This power is from a level 1 creature in MM2.
Tendril Flurry (standard; recharge 5,6) • Psychic
Ranged 10; targets one, two, or three creatures; +4 vs. Reflex;
2d4 + 1 psychic damage.
Having 3 attacks for a potential of 6d4+3 would be pretty potents on one creature.
Additionally, it does not contain wording such as "make 3 attacks" or damage of "per hit".
It's also possible that the attack, whether against 1, 2 or 3 creatures is meant to be resolved with 1 roll and if thats the case that would support only 1 attack per target.
Additionally, you have monsters in the MM2 like the weretiger that specifically calls out making 2 attacks, albeit melee (without target limitations). With no target limitations, you can make 2 attacks against any number of targets up to 2 because thats how many attacks you have. With a target limitation without a number of attacks, it would indicate that you could make 1 attack up to the specified number of targets otherwise they would say you get X number of attacks without specifying targets since by default you are limited to the number off targets equalling the number of attacks.
Hope that makes sense...