The math is pretty complicated but as usual, BruceZ clarifies. Probability forum link. The simplified formula posted by Qui-Gon is a more straightforward approximation.

From my perspective, I just go by the basic principle that pocket pairs are fairly unusual (16:1 against any particular playing being dealt it), and adding in the fact that most pairs, 77+ will be raised at whatever shorthanded table you're talking about. So the lack of a preflop raise on a 333 flop is really positive for AK.