Below is a final position from one of my blitz games.
You can just stop now and try to find the mate in three or read on. (The solution is not given below.)
After the game was over, I took some time to look for the fastest mate possible.
There are (obviously) a lot of mates here, and I just kept trying to find the fastest one that I could.
Without an obvious guide (like the mate in 3 in the thread title), I was pretty sure I nailed it with mate in 4.
Here is what I thought to be best:
1. ... Kd8
The sacrifice denies the king e8.
2. ... Bxe8
Now, the king wishes he had access to e8.
Kc8 4.Qc7 Mate
I honestly thought this was the fastest mate. There is in fact a way to mate one move faster!
White To Move ... Mate In 3 ... There is only one correct first move to do it.
Have at it !!!