Shoot the ground. The reason for this reckless move is because after you shoot the ground, Mr Grey will then, logically speaking, go for Mr White, who is the biggest threat here. If he kills Mr White then it is Mr Blacks turn again, where he can start aiming for Mr Grey. If he misses, the Mr White is, again logically speaking, gonna go for Mr Grey, who is still the bigger threat. By shooting the ground, Mr Black makes himself less of a target for the oyher two, who will focus on each other until one is dead