3 Betting the Flop in Position HU

[QTip]

It occurred to me that I can't remember that last time I 3 bet the flop with a hand I thought was best on the flop when I was HU in position. The only time I think I've done this is when I want a free card, and I believe the opponent will give one to a 3 bet. Obviously, this is a leak that would need to be fixed if I was playing against observant opponents. However, at party 2/4, this doesn't concern me. If I think I have an edge on the flop, and I've been c/red, I always wait till the turn to pop it then after a card has come that I think has not helped my opponent. However, before I see that card, my plan is always to pop the turn unless I have reason not to do so.

Some have said to 3 bet this flop if I think I'm ahead. I don't see why. Please explain.

Are they other examples in a game in which people are not observing your tendencies in which 3 betting the flop HU in position would be best?