Personally I think 3d4+int is too much damage for magic missile. I think the Invoker power that has 3 targets is pure cheese.
What about:
Target one or two people. One target = 2d4+int damage. Two targets = 1d4+int damage each. It can be used as a ranged basic but then it can only have one target.
Alternatly, because I like the three target idea, but I think it is too powerful:
Magic Missile Wizard Attack 1
You launch a silvery bolt of force at an enemy.
At-Will ✦ Arcane, Force, Implement
Standard Action Ranged 10
Target: One, two or three creature within 3 squares of each other
Attack: Intelligence vs. Reflex
Hit: 1d4 + Intelligence modifier force damage. If you only have one target, increase the damage to 2d4 + Intelligence modifier.
Increase damage to 2d4 + Intelligence modifier at 21st level or 4d4 vs a single target.
Special: This power counts as a ranged basic attack. When a power allows you to make a ranged basic attack, you can
use this power. As a ranged basic attack you can only have one target.
That is maybe a bit over the top in utility for Wizard at-will power, but it shouldn't be too unbalancing.
"Within 3 squares of each other" is intended to mean you can hit all the x's here:
xox
ooo
xoo
But not here
xoox
xooo
oooo