He should shoot at the ground.
If Mr. Black shoots the ground, it is Mr. Gray's turn.
Mr. Gray would rather shoot at Mr. White than Mr. Black,
because he is better. If Mr. Gray kills Mr. White,
it is just Mr. Black and Mr. Gray left, giving
Mr. Black a fair chance of winning.
If Mr. Gray does not kill Mr. White, it is Mr.
White's turn. He would rather shoot at Mr. Gray and will
definitely kill him. Even though it is now Mr.
Black against Mr. White, Mr. Black has a better chance of
winning than before.
Lover got it right, even though it not 100% its in the ball park. lol