If we are going to debate, let's not cherry pick. The spell description says
"You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range. A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several."
The bolded parts strongly suggest that the damage is individual. "Simultaneous" doesn't actually have any implication regarding whether the damage is singular or multiple instances, just that they happen at the same time.
Well, I wasn't cherry picking as you seem to think, I was only pointing out that, even though each dart deals damage, it all hits at the same time. That was my point. Like my falling example. If you fall you will
very likely suffer multiple injuries, even if minor, which would all be damage. But the game groups it all into one total.
Do you tell a player hit by multiple magic missiles the damage they take from each separate missile, or just tell them the total?
For example: a PC has 8 hp left and is hit by all three missiles (which we
know strike simulatenously). You roll damage and get 3, 5, 2 total. If you say the 3 and 5 are applied first, reducing the PC to 0 hp, the last 2 would be an auto fail death save. But what if you told them the damage was 3, 2, and then 5? Well, now they are still at 0 hp, but no auto fail death save.
If you roll the damage "in order" and tell them each individual damage, you are then forcing a random order on something that happens simultaneously. After all, with those damage rolls, only saying the 3 and 5 (or 5 and 3) would result in a failed death save. Starting with the 3,2 or 2,3 or the 5,2 or 2,5 combinations would NOT result in a failed death save.
So, random chance of how you told them the damage results in just a 1/3 chance of a failed death save in this case.
Which, IMO, you shouldn't do. You give them the total
because all the damage takes place at the same time. The PC takes 10. No failed death save.
If someone wanted to invent "Delayed Blast Magic Missile", that is just like normal magic missile except that the missiles fire off in order a nanosecond apart and not simultabeously, what level would you make that? If higher than 1st would you up the damage? What if they only wanted them to do 1hp each and stay first level?
The game doesn't function in time like that, so I would tell the player not to be silly and stop "working the game".
You still take damage 3 times, not once. Feel free to change the rules however you want of course. Personally I just wouldn't use MM against a dying PC to kill them off if it's not what the group wants out of the game.
No, you take damage ONCE because it all happens at the same time. (See my above point.)