Usually there are 120 combos of top pair on the flop. When the top card is paired, there are 88 combos of trips. That's only about a third less and a fish would have all those Ax hands. So you can't just look at an AAx flop and say he couldn't have an ace. You've got to play this like KK on an A high flop. It's a little better than Axy but not much.
edit: Well if you know villain is loose-passive you can get three streets of value, but then folding to a raise is even more important.