What's the meta of Twisted Treeline?

Triple bruiser with a jungler dominates low elo, since low elos do better with the exp lead.

In higher elo, adc / support bot with a bruiser top is meta. Support takes exhaust and ignite.

It is actually quite complex and deep of a meta. It is more open than SR because of less team comp limitations.

However somethings to note.

Bruisers are very good. I myself have had extreme success playing Shyvana jungle on TT. People like Darius and Olaf are also very strong.

CC, Tankiness in general and dueling power are good. They often dwarf burst in this mode. As you lack the number of people and champion variety from 5s, it is harder to escape CC and harder to kill tougher champions. So being able to duel people is important.

People I see a lot are Jax, Gragas, Darius, Shyvana, Poppy, Morgana, Ekko, Yasuo, Mordekaiser, Lucian, Graves, Jinx, Sivir and Teemo.

Source: I play it. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2318509315/48865711?tab=overview

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2318475441/48865711?tab=overview

For 3v3, most bruisers work pretty well. Any mage that likes to build Liandry's and/or Rylai's as core also performs well.

For 6v6, siege mages and long ranged adcs dominate. Just mash those long range spells, build some pen, and you'll win most games. Personally, I think TT's map is too small for 12 players.