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.
And “Damage Rolls,” PHB, 196:
If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a wizard casts fireball or a cleric casts flame strike, the spell’s damage is rolled once for all creatures caught in the blast.
I mean, they even provided a link to sage advice where Crawford answers that specific question.
I’m sorry but this is just wrong. From PHB 2014:
Each dart deals 1d4 + 1 damage and there are 3 darts, so 3d4 +3 damage.
Nope, here is RAW:
Magic Missile:
And “Damage Rolls,” PHB, 196:
I mean, they even provided a link to sage advice where Crawford answers that specific question.