Can anyone help me spot the mate in 3?

Hi,  I played this game recently, and afterwards I used the computer analysis. The computer says this position (begining with 21. Re8) is a mate in three.  Since I'm trying to improve I thought I would see if I could find it, but I couldn't. Can someone find it? I understand that blacks only move is 21.kf6. But after that I am unsure. 22.Qf7 looks like the logical next move, but after that I don't see the mate.

Qxe5+ loses the queen to the knight so maybe Rf8+

It is black to move and there is no mate in 3:

a) White king is safe.

b} Black pieces are passive and uncoordinated.

c) The only black's legal move is 1. ... Kf6 and white can continue with checks, preventing black from any attempts.

d) It is actually white who can deliver the checkmate (1. ... Kf6 2. Qf7+ Kg5 3. Qf5+ Kh4 (3. ... Kh6 4. Nf7#) 4. g3#).

Regarding d) I meant that white had a mate in three, not black. Maybe it was unclear.  But thank you, that mate in three was driving me nuts. Cheers

22. h4,  threatening Qf7 mate, may also force mate in three. 

22. ... Nd8   23. Qe5+  Kg6   24. Qg5

22 ...Kg6  23. Qf7+ Kh6   24. Nf5#

22. ... any other move   23. Qf7#

Thank you. I hadn't considered there were more ways than one to mate in three, very instructive. 

Not looking at any other comments or computer analysis...

Kf6 Qf7+ Kg5 Qf5+ Kh4 g3#

Or

Kf6 Qf7+ Kg5 Qf5+ Kh6 Nf7#

How does black play Nd8 (or your "any other move")? He's in check.

I also assume you mean Kf6, not Kg6

What about:

 ...... Kf6

Qf7+ ..... Kg5

Qf5+ ..... Kh6

Nf7 #

Or

Qf5+ ..... Kh4

g3#

I can't mate inthree w/ 2.h4 and if black sacs his knight to protect e 7, I  don't see a mate in 3.

Splane is right. The thing is the sequence begins with white playing 21. Re8 and black has to play 21. Kf6. So all his combinations starts with 22. and are mates in three.