[edit] sorry, I thought you were White. 3 ... exf4 is a mistake, read below
Well, you made the 1st mistake, after 3 ... exf4 you have to play e5.
Black is not really supposed to take the pawn. after 4. e5 the Knight has nowhere to go except back to the starting square, and White has a really nice lead in developement. One of My all time favourite games went like this:
analyze this (please)