Generally, you run a trilane to get guaranteed farm and protection on one hero who is very gold reliant. This way, you get him into the game more quickly than if he had to contest for farm. You don't usually want to trilane with a carry hero who needs levels to be effective like Shadowfiend or Drow, since the carry will split a lot of experience with his supports. Good carry heroes for trilanes are Luna, Gyrocopter, and Lifestealer who can all be very effective heroes in the midgame with only ~5000-6000 gold and one or two key items.
You can also run a defensive trilane with a very hard carry hero like Faceless Void, Spectre, or Medusa in what's called a "four protect one" strategy, but this is not very common recently because it doesn't do well in 3v3 lanes and is weak against aggression.
Priorities in a trilane (higher is more important):
- Your carry is getting as many last hits as possible.
- Your supports are not wasting their time- if they aren't doing much in the lane, they should be pulling, stacking, or ganking another lane.
- Whoever you are laning against is not getting last hits (if 3v2) or experience (if 3v1)
- Killing the opponent and then pushing the tower.
A lot of new players make the mistake of thinking that a trilane is for getting kills and do things like Juggernaut trilanes to get 3-4 kills in 15 minutes. This is good, but 3-4 kills is about 20 last hits, and these new players ALWAYS miss more than that. Each lane has about 80 creeps spawns in the first 10 minutes. In a trilane, your carry hero should get at least 60-70 of them and should be aiming for perfection. Killing the opponent if he gets too far out of position is just a bonus. The carry player also needs to deny as much as possible to keep the creep equilibrium static- autoattacking will push the lane and give your opponent unnecessary experience and gold.
Another common mistake is that supports worry too much about harassing the opponent and not enough about the overall game. They need to pull and roam so that they secure experience and gold for themselves as well. If your enemy is contesting the lane, they should stick around until it's secured.
There are a ton of different variations of trilanes- here are the more common ones:
- A typical safelane trilane where you're just passively farming a carry and your supports are using the jungle or zoning your opponents out of the lane. Generally, you need one support to harass for each opponent in the lane with the carry ready to help at any time (voicechat is crucial).
- An offensive trilane where you try to have a more powerful killing lane than your opponent's trilane (triple stun, debuffs, auras, etc.). Here, it's important that your carry be defensive (high base armor and/or health) because he's usually the easiest target to go on.
- If you run your trilane on the offlane (not jungle side) and only have one or two opponents in that lane (plus the enemy jungle), your goal should be to try to get a tower kill or hero kills as soon as possible, because it's a lot harder to keep the enemies from getting last hits and experience (they can pull and have a close tower to retreat to). This is a lot easier if one of your heroes is a good pusher. After you do this, you gain a lot more map control and it usually forces the opponents to start roaming.
My personal favorite way to run a trilane is with one strong ranged disable and one strong slow- this gives the trilane the core killing potential it needs and anything else is just a bonus. Double stun lanes (one ranged and targeted to start things) also work well. Without this, punishing out of position opponents is a lot more difficult.
- Lifestealer and Vengeful Spirit
- Luna and Tidehunter
- Dragon Knight and Rubick
- Sven and Venomancer
- Naga Siren and Jakiro
- Chaos Knight and Leshrac
Your third hero should be a strong roamer or potentially a jungler, depending on the strength of the lane you're up against. If you're up against two or three opponents you'll want more burst damage like Lina, Keeper of the Light, or Visage or defensive power like Shadow Demon, Dirge, and Dazzle.
This is a great guide to trilaning in original DotA that helped me when I was learning.