My 3v3 has about a 75% winrate, so I think that may have helped in placement? But at the same time, one of the guys I was with had played 4 3v3 games prior to the matches. (Solo queue is fucking worthless and retarded, for the record. Being "good" at solo queue is not directly correlated with being good at league. I refuse to participate in their broken solo queue league system. My "low MMR" is only because I don't play it unless I try and troll like Zyra jungle.)
As Smurfin said, "Renekton, Wukong, Jax, Jarvan, Zac are all great." Vi works well too, especially if they pick a squishy. You want some high-CC and high damage output. It's nice to have someone with the kit to solo Vilemaw, as well. Jax with some beef and a Blade of the Ruined King can solo him at about 13 minutes into the game, while your other teammates continue pressuring the map.
High-mana cost AP carries are also OP as shit because of the mana regen passive on Treeline, so like Zyra, Annie, Cassiopia. My AP Carry found Annie to be particularly ridiculous because the stuns were just so tide-turning in team fights.
Junglers can have a bruiser or another jungling champ, like Amumu/etc. Base it on your team more than on "Tier lists." In one game I chose Nautillus who is apparently D-tier (we were fighting 3 melee champs so the stun was incredibly useful) and I ended up going something like 1/1/14. If you have a squishy without many defenses or escapes (such as Zyra) it helps to also have Kayle jungle because her burst is HUGE and her ultimate is very good as well.
All in all, "Pick 3 bruisers" is over-simplifying it, but you get the gist. Pure carries like Master Yi or an ADC are generally harder to pull off in Treeline, where a balance of pure brawling and some utility/CC comes into much more value.
