i was playing online when i beat someone in three moves but my rating stayed the same because it said i only moved once!!!

1 f4 e5

2 g3 exf

3 gxf Vh4 #

This is chekmate, but how can the bishop stand on g2 ??

I would consult with the operator of this site.

This isn't mate.  Not sure how it got to this positon, but it isn't mate

King can go to f1, then either block the ck with Kf3 or Bf3.

Looks like this is the game he's talking about:

http://www.chess.com/echess/game.html?id=8142587

Since this game took place over a year ago, Chess.com still had the policy that games that ended before 4 moves were played did not affect one's rating. This isn't the case now.