The one thing I recommend in 3v3 is to play some compositions such as one APC with a lot of control and a safe farming phase (Morgana, Graggy, Ziggs), one bruiser that can be an ADC pick or at least that can hard carry end game (or the contrary) such as Jayce, Vayne, Trynda, Trundle. And for the last role, with my teammates we figured out that something really cool is to play junglers that can be viable as support (or the contrary) in 5v5 games, such as Nunu, Nautilus, WW, Cho'Gath... Even Morgana works!
And so, with this kind of compo, you can take a smite, support your ADC/bruiser and everytime you can, you go in the jgl to take some farm, or go and gank the other lane while your ADC/bruiser plays safe.
Using this technique made us climb to Plat IV in 3v3 last season, and it is also working this season (Gold II last week)