It's 1-(chance you miss all three cards) = 1 - (44/50)*(43/49)*(42/48) = 32.4%

This pretty much.

But if you want to know the probability of hitting exactly one pair (the above formula disregards the fact that you can flop more than 1 A or K), what you really have to look at is the amount of flop combinations that give you only one A or one K (duh). There is 50 * 49 * 48 / 3 * 2 * 1 = 19600 flop combinations. Then there's six out of 50 cards that give you a pair, meaning the other 2 cards can not be an A or K. There's 44 * 43 / 2 * 1 = 946 such 2-card combinations. Since there's 6 aces and kings left (three of both), that's 946 * 6 = 5676 flop combinations that give you exactly one pair.

5676 / 19600 = ~28.95% chance to hit exactly one pair on the flop.

But the other formula also includes hitting things like two pair, trips, etc.