Must I declare the target of all magic missile projectiles at once?

Question

My sorcerer casts magic missile. He has 3 projectiles he can target up to 3 creatures. Must I declare the target of all missiles at once then resolve damage / shield / immunities, or can I shoot one, roll damage, shoot another (at the same target or another target), roll, and so on?

Must I declare the target of all magic missile projectiles at once, or can I shoot them one at a time (in the same action) resolving damage for one missile before using the next one?

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_{Looking for textual RAW or RAI references. It is assumed that the DM can set a ruling one way or the other disregarding the text, so we don't need to address the Golden Rule.}

Answer 1

Yes, you must declare all of the targets at once.

The missiles are simultaneous:

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.

(PHB, p.257 , emphasis mine).

Since the darts all strike at the same time, there is no way to wait for the results of one missile before declaring the target of another.

Answer 2

Declare all targets before resolving damage.

Per the spell description,

The darts all strike simultaneously, and you can direct them to hit one creature or several.

Emphasis mine. This would mean that you would need to choose all of the targets before resolving any damage.