On the first card of the flop: 48 cards are bad for you out of the 50 unseen cards.
On the second card, 47 out of 49 cards are bad for you
On the third card 46 out of 48 are bad for you.
The formula you then get is: 1 - (48/50)*(47/49)*(46/48) = 11.75 %, which translated into odds is about 7.5 to 1.