Same rules as always: 

1.) White must mate in 3 moves.  No exceptions.  Assume that you have 1m30s on the clock and each move takes you 30s to make (whether it actually takes you 30 seconds, 5 minutes, 10 minutes or 10 seconds, just assume it takes 30s).  If you don't mate in 3, it's game over and black wins.

2.) There is only one key move to obtain mate in 3.  (If you find others, please let me know )

3.) No first key move will result in a check, a capture, or involve the promotion of a pawn.