WHAT TO DO WHEN THE GAME STARTS
ALWAYS MAKE VILLAGERS.
I cannot stress this enough, because I know people that will only make 5-10 villagers/settlers a game. Next game, try NOT TO STOP VILLAGER TRAINING, and observe your muchly increased production. Quickly gather your food crates first to queue the first villager as fast as possible. Only stop making villager when you have enough resource to advance to Age II [800 food]. Try to make more Town Center if you reach Age III to train more villagers. The best amount of villlager/settler to have is MAX, train villagers until you can't train anymore).
SET GATHER POINT
Click your Town Center to select it and right click on your most needed resource, usually animal first for food. This will make your villagers do what they need to do right away after they are trained and not to stay idle next to your Town Center doing nothing.
HUNT
Except for Japan, all civilizations can hunt animal to gather that 800 food to advance to Age II. Hunting got fastest gather rate [faster than picking berry bush]. Don't ever think of making a mill/farm in Age I again and hear everyone laughing at you because it costs extra 400 wood and still lower gather rate than hunting. This also applies to mining [instead of plantation].
HERD
Shoot the animal toward your Town Center so they run toward your Town Center. Wait 10 seconds and shoot a new animal for one time then go back to the first dead animal [so the animals run even nearer toward your Town Center but not falling dead all - dead animal will decay and decrease the total food you can gather from them]. The closer to your Town Center the animals are, the less distance they have to walk, the less dangerous it is to hunt. You can garrison your villagers quicker and save them from later raiding.
PAY ATTENTION TO THE VILLAGER IDLE FLAG
When your villagers are not gathering, you have no economy=no army=no victory.
EXPLORE/SCOUT.
Click to select your Explorer then shift click him around your base to see nearby hunts/mines and to hunt treasures, around your enemy base to see his location, and also his nearby hunts/mines. Look for what units he is making, what his shipments are, where his army is, where his villagers are working.
SEND YOUR CARDS.
First card: always a villager card in Age I [except for Japan, Russia] because you have nothing better in Age I than developing the economy and prepare for Age II battle.
Second card: should be saved and sent after you reach Age II. [could be a military shipment if you want to be aggressive early or you are rushed, could be a villager shipment to furthermore develop a solid economy, could be a resource crate shipment so you have instant resource to make more units].
A shipment can make much different in your economy and military. Remember to take advantage of it.
RAID.
Every once in a while, make 2, 3 or so hussars and send them to your opponents base. Use what you know through scouting, look for dead animals sign on the map, search around trees/mines. Avoid defensive buildings and troops, just kill their settlers. When their military comes after you, run away, save them for the next raid.
Also watch out for enemy raid on your minimap, build house around to expand your line of sight, if you are a bit far from Town Center, DO tower up your key gathering point. DON'T leave your villages out to dry with no cover if you know your enemy is raiding (eg. send 5 pikes to babysit them against the cav harrssment you know is coming).
MARKET.
DO get Market economic upgrades. Hunting first, mining and wood later if possible. Market upgrades can give you significant lead in economy over time.
DO get Villager hitpoint [HP] upgrades in Age II (This can save your villager life if you are under raiding).
GARRISON 10 VILLAGERS IN TOWN CENTER.
Run your villagers into your Town Center when you are under a raid. 10 villagers inside Town Center would give you max attack [90 attack] and can help you kill a subtantial amount of enemy units. More than 10 doesn't increase the attack, use them to gather resource on the other sides where your enemy can't see them or reach them. Also eject your villagers out of Town Center as soon as the threat passes so your villagers can continue to work on your economy.
MINUTEMENT.
DO use Minutemen if you are overwhelmed by a rush. They are emergency unit and if assisted by Town Center fire, they can help you to push away most early rush.
DON'T use Minutemen if enemy is still too far from your base that he can just run away from the minutemen and come back when their HP drains to zero.
Check out our Unit Counter section to know what kills what. You can never win any battles if you are making Longbowman when your enemy is making Hussar.
MICRO MANAGEMENT.
Shift Click: each time you click to select a villager, then click a mine, then shift click another mine, you create a queue of tasks for that villager to do, they will finish the first mine and move to the other mine without you watching and controlling them all the time. Save your time, increase your efficency whenever you can with shift click. Use it to scout, shift click your Explorer around the map, shift click your Hussar on different villagers when raiding, shift click your villager to build a house then moving to cut a tree. Shift Click when sending resources to your teammates can let you send 5x faster with 500 resource each time instead of 100.
Use Hotkey: explore your Hotkey in Options/Hotkeys, you can create group of units by hotkey, quickly select Explorer, barrack, stable... by hotkey insteading of moving your mouse around the map to find them then quickly train units from your barrack, stable without looking at them while you are fighting.
Attack Move: the Attack Move hotkey can be customized in Options/Hotkey/Command Hotkey. with small group of range infantry/ range cavalry can give you more kills, select your units and press Z + right click on the enemy's army. Attack Move will move your attack to another unit after the last unit died. It prevents overkill [which means all units keep attacking a dead unit even after it's already out of hitpoint]. It doesn't work with longbow man or large groups of hand cavalry, they just act weird or having pathing problem.
Hit and Run: in Age of Empires 3, there are some units with longer range than other units. That means longer-range unit [skirmisher/longbow/dragoon...] can attack without being counter-attacked by the shorter-range or melee unit [of same speed]. Attack your enemy unit [skirmisher vs musketeer, dragoon vs hussar] and run away before your enemy unit can reach you. Keep your units alive while leaving your enemy with death and defeat :).
Group Unit: by group hotkey, group your Hussar while raiding to control them easier, when fighting, use group to max out your counter, control your Hussar unit group to attack Abus Gun while your Skirmisher/Longbow are targeting Janissaries.
Timing and Positioning: it's important to know when and where to attack/defend. If you scout that your enemy has trained 10 Longbow when you are having 5 Musketeer, it's not a good time to push him, build a stable to train Hussar first. If your enemy is attacking your 5 Musketeers with that 10 Longbow, don't suicide them to fight blindly, retreat back to your Town Center for better position, then move your Musketeer close to the Longbow [because Musketeer melee/hand attack is stronger than Longbow's, but their range attack is weaker]. With Town Center fire [10 villagers inside] and your Musketeer melee, you would be able to hold off those Longbows easily. Otherwise, your Musketeer would die all for nothing.
STRATEGIES
RUSH is a popular strategy in supremacy standard game that includes putting priority in developing military and attacking early in Age II. It can be a hard rush when a rushing player typically have a weaker economy in favor of a stronger army via unit shipment and attack as soon as possible. It can also be a delayed rush when player takes economy shipments and stabilize their economy before hitting the enemy. Ideally most rushes are executed within the first seven minutes of gameplay (or even less with hard rush).
Advantage
By attacking early with a bigger army, you can harass and disrupt your enemy's economy. Rushing gives you the momentum in the game: since you are the aggressor, you can get more map control, pressure your opponent off outside resources.
Disadvantage
If the defender manages to hold off the rush [by smaller army assisted by Town Center fire, minuteman], the longer a match draws on without the rush inflicting significant damage, the more likely it is that a rushing player will not be able to seize victory due to their slower economy.
Popular rush civilizations with strong shipment of military and early cost effective yet powerful units are Iroquois, Ottoman, India, Aztecs...
RAID is to search and kill your opponent's villager, as many as possible. Since Age of Empires III games are eco-heavy (i.e. eco is just as important as military), one of the most important/best/viable strategies has always been the raid. Raiding basically is disrupting your enemies econimic growth, while yours is steadily devolping at the same time. If you do this right, you can always have more settlers, more military and more resources than your opponent.
One of the most effective units to raid is Heavy Cavalry [Uhlan, Hussar, Cossack, Oprichnik...] with its speed and direct deadly hand attack [villager has range resistance]. A raiding party usually includes 3-5 cavalry to reach and kill and run away quickly if confronted with anti-cavalry units or Town Center fire.
FF [FAST FORTRESS] is to advance to Age III as fast as possible, usually via resource shipment to take advantage of Age III powerful shipments of military and economy. It includes naked FF [usually is the 700 gold shipment in Age II to go directly to Age III] and semi-FF [send a villager/military/resource shipment, produce a batch of miliatary to defend or raid...before going to Age III]. FF is a popular move in boom/turtle strategy with these civilizations: Ottoman, German, Portuguese, Spanish, Dutch...
BOOM is to invest first in economic growth [usually by resource/villager shipment/economy technology upgrade/economy upgrade shipment...] in early game and to overwhelm your enemy later with more resources which lead to more units and better upgrades.
Advantage
If a boomer is left alone and allowed to boom, in the late game he will become unstoppable due to superior economy and military.
Disadvantage
Booming is very vulnerable to a rush. It is crucial to know the right them when to stop booming and when to mass military.
Civilizations with strong boom includes Japan, British, Russian...
TURTLE involves investing in defenses early, such as walls, outpost and Town Centers as well as a small military force. The idea of the turtle is to protect your base while the aggressor clashes against your defenses to no avail. Thus, all of the resources the enemy spent on the rush are wasted, while you spent fewer resources protecting yourself, and now have the advantage.
Advantage
The turtle makes your base safe and secure form attacks, allowing easier raiding and safer economic development. It also protects you from an early, aggressive rush.
Disadvantage
The investment in static defense [outposts, wall, fort, Town Center...] can be expensive, and if your defenses are cracked, you're in a lot of trouble, as you don't have big army inside your base, leaving your economy exposed to the enemy. Protecting yourself inside your base is only good in the short term. Eventually you have to expand outside the safety of your base in order to get hold of more map control and natural resources, or your enemy will take them all, leaving you at a significant economic disadvantage. This is also the case if your enemy doesn't attack at all and boom instead, leaving you behind economically.
Dutch, Portuguese... are popular civilizations for turtlers.
BEING FLEXIBLE/ADAPTIVE is what usually your strategy should be. Strategies can be mixed and adapted to diferent situations/maps/players style.
The ability to scout, predict and decide when to be launch an attack at the right timing to deliver the most damage or to withdraw your troop to avoid fatal defeat is important to your strategy.
An all out rush isn't really viable without a sustainable economy. You will need resource/villlager shipment to boom if your rush is screwed against a turtler.
Turtling and booming often go hand in hand, but they don't need to. If you're confident you're not going to be rushed, booming without investing a great deal of resoures into walls and outposts will put you in a better position later. Also, if two players are determined to BOTH boom, the one who spends less on units and defenses early on will likely be in a stronger position.
Turtling without booming, on the other hand, is typically the response of a player in a weaker early position, with the hope that at some point, you'll regain the advantage, and be able to take the offensive. This is a risky proposition, because even the weakest rush economy will outperform yours if you have to garrison large numbers of your workforce.