ezo
Hero
I disagree (obviouslyTechnically magic missile does do damage to more than one target at the same time, since it specifies that each missile hits simultaneously. So by 2014 RAW it did in fact fall under the rules in the damage rolls section cited in the OP.

In a case such as magic missile, it doesn't make sense. At 1st-level, you get 3 missiles:
1 target: 3d4+3
2 targets: 2d4+2 and 1d4+1
3 targets: 1d4+1 each
Fireballs and similar spells deal equal damage to all targets, so all targets take the same damage. Magic missile due to its very nature, usually doesn't.
You could roll once of course in the case of 3 targets, but rolling (d4+1) x3 for the damage against a single target doesn't make sense as written.
Anyway, JC's interpretation is just as valid as mine, so I would go with whatever a DM does for their table.
Also, consider a spell like eldritch blast. At 5th level, you can attack multiple targets. Do you honesly believe people roll damage once and use the same amout for each beam that hits?
Regarding Concentration checks---a single check IMO. It is one spell, one source of damage, one check.... If you hit a bless-casted cleric with a 1st level magic missile, are they making 1 concentration check or 3?