[Homer]

This could very well be AA.

From your opponent's perspective:

Preflop - I have AA, I raise

Flop - He raised me, think I'll checkraise the turn

Turn - Hmmm, I turned the nuts, think I'll wait until the river to pop him.

River - Oh crap, I don't want this to get checked through. I think I'll wuss out and bet it myself rather than going for the checkraise.

That being said, since the guy is a maniac (based on the title of the post) I'll guess he has 34, 48, or 89.

I would have called the three-bet just as you did.

-- Homer