First off, welcome Rhombustible to DreamCancel. I think Ginorm said a lot of what I would've suggested for getting into the 3v3 format. I agree with just jumping right into 3v3, but would advise slowly trying out characters that appeal to you, one by one, after you've got at least one team you're comfortable with.
In sticking with this game, there's a good chance that you're going to have several teams, several back-up teams or just characters you like to mess around with; the more characters you know and learn, the more your team(s) dynamic will change to your benefit.