Well, I think the simplest explanation is he has a 3 in his hand for trip 3's. Of course, he could always be bluffing. But in an unraised pot, you did the right thing by folding in the end, I think. People will limp in with lots of stuff UTG on a short-handed table, he could have had A3 or any suited connector hand with a 3 in it.

Of course, I fold too much anyway! Without knowing the tendencies of your opponent, the safe thing to do is just fold and let the small pot go.