Target: One, two, or three creatures
Attack: Dexterity vs. AC, three attacks
...
Pick one, two, or three creatures.
Against each of those creatures, make 3 attacks.
Not quite. You make three attacks with this power, period. Since it has one, two, or three creatures, you can choose how to split those attacks up, in the possible combinations Black Knight Irios has mentioned. You can't fire three times at each target, just like Twin Strike doesn't let you fire twice at two seperate enemies - it lets you fire twice, and you can either split it between two targets or focus both attacks on one target.