How do I deal with Amumu.

As anyone.

Please respond.

Aside from stealing his blues (ward your own using your supports free 1 min one) and breaking his back in his own jungle?

Like the poster above me said, DO NOT let him have his blue. Invade him and take his blue and yours as well. Also if his CC is a problem there is always Quicksilver Sash. I believe it builds into some scimitar item that gives AD. If his ganks are a problem then ward the river and you can even ward tribrush if you need to. In team fights be ready for his ult, it is going to be used. Magic resist can help as well.

A good disengage counters him in teamfights. Janna, Syndra, and Tristana come to mind

And to counter in 3v3?

Amumu is OP atm, based on win rate according to lolking
That being said, to deal w/ him, besides shutting him down hard early, spread out a bit before teamfights start, so as to make it hard for him to ult your whole team, and beware of diving his team if he's in the middle of them, as you may very well get ult'd and counter-initiated on.

dodge his bandage toss and your solid most the time.

Counterjungle and juke his bandage toss

His damage is not as amazing as some think. watch for his b toss and use range to your advantage kite him around and take his blue buff and its gg for him.