Amumu is one of my favorit champs on 3vs3 cause he is a tank killer with his w-skill, and his e-skill has a nice damage output at the early game against squishys. He has an incredible cc with his w and r and can kill the dragon really fast. You should go on the botlane with an ad offtank like xin or somone else who is great with a stunner on his lane. Your role is to stune and to deal magic dmg also you can turn a fight easilly with your ultimate. You can do also nice ganks on the top-lane. like that i have a 75% win raito at 3vs3 rankeds with my team.
Have much fun with my amumuguide!