Rainbow flop, no flushes, no straights, with betting like that, I'd suspect a scared 2 pair out there. With an over-set, a player would like to keep other players in, especially if some one else is building the pot betting into you. Now, consider how this would have played out if you were in an earlier position. With your set, a $36 bet, would you raise, or just call with players behind you? I would find it very tough to lay down a set with no threat of a straight & a flush. However, this is an excellent way to develop "instincts". Laying down a hand is usually just a "small" mistake & it takes as much courage to fold as it does to call in this situation.

Just my take on it! Good luck!