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 safe trilanes are those who can all be very effective heroes in the midgame with only ~5000-6000 gold and one or two key items such as Armlet or BKB.
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.
- Whoever you are laning against is not getting kills (if 3v3), last hits (if 3v2), or experience (if 3v1)
- 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.
- 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 safe 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 stack, 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 against an offlaner or no one 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).
- 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.
Finally, there's a safelane trilane against an opponent's offensive trilane, or vice versa. This is one of the most complex laning scenarios in Dota and it's very hero dependent and guaranteed to be razor's edge. Here, there are a few things you definitely need to worry about in this scenario:
- Strong supports. The strongest trilaners have good base stats, strong AoE debuffs or buffs, and the ability to kill enemy heroes who are out of position without overextending. For example, Naga Siren is an incredible trilane hero because of her 6 base armor, lengthy Ensnare, and AoE armor debuff/nuke Riptide. Rubick is also great: his Fade Bolt reduces enemy autoattack damage and his Telekinesis is excellent for pulling enemies who are slightly out of position even more out of position. Defensive heroes who can do the opposite are also valuable, like Shadow Demon with his Disruption or Abaddon with his ability to remove stuns with shield and heal. Other good trilane supports: Dirge, Visage, Tidehunter, Venomancer, Bane, Vengeful Spirit, Earthshaker and many more. Each of these have some combination of the above traits that you should be able to recognize, and they get them at low levels.
- A carry who is somewhat gold dependent, but not too level dependent, and is able to contribute to fights early on. This hero is going to be farming, but in a 3v3 lane will be forced to split experience. One of the most important attributes on a carry is their base armor/survivability, and they should almost always be completing a Ring of Basilius with their first 325 gold in lane to boost that for themselves and allies even more. If the carry is able to contribute to kills with a stun or powerful slow, that's also huge. Alchemist has low armor but is a great trilane carry because of his offensive power with his Concoction and armor debuff. Lifestealer is very similar: he makes up for his low armor with a built-in BKB and an incredibly powerful slow. More traditional trilane carries like Gyrocopter, Luna, and Slardar are tough to bring down and have powerful clash abilities.
- Good gankers in your solo lanes. Having heroes like Puck, QoP, Clockwerk, Bounty Hunter, Weaver, Nature's Prophet, etc. who can dominate lanes and easily kill underleveled supports is almost a necessity if you plan on 3v3 laning. You can also be more greedy than usual when choosing these heroes because it's very difficult for the enemy supports to rotate and gank when contesting a trilane vs. trilane.
My personal favorite way to run a trilane, regardless of what we're up against, is with one strong ranged disable and one strong slow- this gives the trilane the core killing potential it needs and anything else on top of that 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
- Alchemist and Tidehunter
- Sven and Venomancer
- Luna and Visage
- Lifestealer and Naga Siren
- Slardar and Lich
- Gyrocopter and Jakiro
- Dragon Knight and Rubick
- Skeleton King and Leshrac
- Chaos Knight and Ancient Apparition
Your third hero can fit into the puzzle however you need based on how the matchup and overall team composition is looking. Depending on the strength of the lane you're up against, it can be a strong roamer, pusher, or potentially even a jungler, and if it looks like you're in for a 3v3 lane you can shore up your weaknesses there.
Finally, this is a great guide to trilaning in original DotA that helped me a ton when I was learning.