I was wondering what the meta is in 3v3?

There are different thoughts on this.

The most successful metas at higher Elos seem to be Support and AD carry top and a beefy mage bot, but bruiser (or AD carry) top, beefy mage bot and some hybrid bruiser or tank jungle is also decently popular.

The ordinary low-Elo meta will be a bruiser top and then something random bot lane, preferably making the team's damage around 50/50 magic and physical damage.

Some ppl argue that you shouldn't be talking meta for 3s, but these are the ppl who just roam and don't lane at all - And they fail so badly.

agreed, but a roaming support can be efficient aswell

no not really, you can get caught and killed so easily, and the map is so small that 2 wards is mostly all you need.
tbh there isn't really a "meta" in 3v3, i've seen a 2 support and a tank team and they work, my point is, as long as the players know how to play the champ then they will have an easy time.
thought, bruiser are dominant in 3v3 right now and high mobile high sustain bruiser especially like lee are very good :P

The "meta" is concerned about jungling. The most important thing on Twisted Treeline is the Dragon and Buffs.
Dragon gives global experience bonus, 280g+25 to the killer, which is like destroying 2 turrets.
The red buff is the same as in 5v5, The green buff (10% aspeed+10% CDR) is meh, The white buff (40% movespeed) is incredibly OP.
The one who controls the jungle will win the game.

This is why Jax, Udyr or Tryndamere are very popular - they are very good junglers. Tiger-Udyr can take out the dragon on level 5 with just Wriggle's. Tryndamere doesn't even need the lantern.

Another important factor is - the walls are thin, champions with "get through" dashes and blinks have further advantage (i.e.Tryndamere again, just spin through everything)

You need to find out a strategy yourself. I won many matches just by picking a good jungler.

This is how you can play until you want to be near the top.

Roaming supports are quite popular among the best 3v3 teams - It's a good way of controlling the lanes and jungle. Having the support also allows for an AD carry in the comp, which will, with proper positioning, outdamage every bruiser from pretty early on.

When you play against players of equal skill as you, the only upper hand you'll have is your team comp. Unless you are the best 3v3 player there is (by a decent margin), your picks will affect your performance quite a bit.

The enemy team having wards is easily fixed with pink wards, which are usually the only wards I use for 3v3s, although I might drop green wards very early when I'm not sure how the enemies' ward patterns are.