If you're in a hilly area and used to a triple, I think a fixed gear bike would be pretty tough for you. Even a single speed. I've heard/read that fixies can be tough on your knees. But they do help smooth out your stroke.
I've pondered converting my (soon to be) old Axiom into a SS to use as a commuter. DH's comment - "You'll hate it at the first hill you hit." Don't know if that's totally true, but DebW's recommendation is certainly a way to go....mimic the expected gearing with your geared bike, and vow to stay away from the shifters for a ride. I don't think I myself would have the leg discipline to ride it "fixed", but giving it a try as a SS might be good.