You'll see a lot of JJ and QQ here IMO. Possibly some AK/AQ/AJ that missed. Slim possibility he has a set or some type of wheel interaction. UTG open tightens ranges though, so it's probably big cards.

It wouldn't be insane to call, but against generic timid low stakes villains, this is probably value more often than it's the bluff. I think a lot of people with just the naked Ac would c/r that flop just to simplify their decision. Flatting and then jamming looks a little stronger to me.

I don't like your pre-flop 3-bet with JTs against a UTG open. This is an example of why. You built a huge pot, hit a very good flop for your hand, and could still be completely crushed.