I've got a 6-Pack and that 3.0 will certainly fit although the chain might rub on it when you're in the 22/34 gear combo. I'd never put a tire that big on the back though unless I was riding a lot of sand or snow.
I've run both the Gaz 3.0 and Mich 2.8 on the front. I like the Mich 2.8, but that 3.0 is just to big and tall for my taste.
I'd say 2.8 max. The only advantage of the Gazzy is sheer volume, and it's not like the Michie is exactly lacking in that department. Otherwise the Michie beats the Gazzy in traction, control, weight, etc. My Timberwolf 2.7 feats my RFX yokes easy. The closest point of contact is actually the seat tube at bottom out - a point most peeps don't think about until it's too late.