Flopping a set is 2 in 50 or 4%, no? 52 cards less the 2 in your pocket = 50, and 2 outs left in the stub = 2/50.
That’s correct if the flop was only 1 card. For three cards you can approximately just multiply that by three.
The easiest way of calculating the correct odds is to figure out the chances of not flopping a set with each card, multiplying those together and subtracting that from one, ie: 1 - 48/50 * 47/49 * 46/48 = 11.8%
Right, of course. D’uh!
WRONG-3 cards are flopped-divided by 25 is about 1 in 8 or around 12.5% 
That is what i basically said, geez:)
Don’t forget:
Sets = Sets - Quads - Boats
This makes the chance to get a Set on flop a little bit lower.
You calculate the chance to get a Set or higher.