texas hold em. possibilty to become the third card when you have a pair on your hand.

52 cards has the whole game. 2 of them i have on my hand (for example a pair of aces) so there are 50 cards out.
5 cards dealed to the table.

i do it this way. seems wrong because i am a math noob :) :

52 - 2 = 50  # two cards are already on my hand
50 / 2 = 25  # because there are 2 aces left outside my hand
25 / 5 = 5   # 5 cards dealt to the table

so for me i have a 1:5 chance to get the third ace. and thats 20%!
my poker calculation programm says to me that there is only a chance of 12%. 
how do i calculate that???