When you use a Power with multiple targets you will make only one Attack roll. The result of the roll will be the Defense that your Attack hist. You will compare this number to the Defenses of the targets that you attacked. If the number is equal to or greater to Defense of the target then the Power will affect that target.
This method of determining hits and misses speeds a Atomic game up dramatically.
A player character uses a Power that can has the ability to affect 3 targets. The player character rolls an Attack roll. The result of this roll is an 18. Now you compare this 18 to the targets Defenses. There are two with a defense of 15, which hit. The third targets defense is 22 which misses. This Power will affect 2 of the 3 targets.