well, there is more variation and it often comes to powerpikes depending on the current patchs balance, but as a rule of thumb you want waveclear, you want either absurd singletarget damage+cc to instantly turn a fight 3vs2, or you want an aoe wombo as the map is rathere small, you have no room to spread out and avoid aoe.
Bruisery champs usually do well, though kitecomps or protect the cary comps do work aswell. The only thing that is nearly alway there is a utillitymage, you rarly see high elo 3v3s without one on each team.
And since the latest changes that introduced aditional towers, you nearly always have a jungler, exept for very specific strats like a janna+cait fastpush.
However, haven't played 3v3 since a bit past the juggernaut patch, given the current balance, adc are probably close to mandatory in tt aswell, with peeling stuuf like poppy or nauti