While I understand why people interpret Magic Missile as making you do three concentration checks, or being three death failures (it makes sense, regardless of it hitting simultaneously it is still three pure bolts of magic), the spell is already one of the best 1st level damaging spells as it is, being a guaranteed source of damage and being able to hit multiple targets. Allowing the three checks is... Too much, I think.
I also question DMs who make the enemy wizard cast magic missile on downed PCs. Not only because it is kind of a dirty move, but because... Why would that wizard do that? They don't know if the PC is dead or unconscious. Would they really waste a spell on a body on the ground when they could instead target the people actively trying to kill them? You gotta take the personality of the npc in mind, and I can't really think of a character who would do that other than "Not even I know what my next move will be" types.