Today, wizards released the Ninja guild option for the executioner assassin.
As one of its powers, it gets poisonous shuriken:
Target: One, two, or three creatures
Attack: Dexterity vs. AC
Hit: 1[W] damage.
Given that this is "Martial, Weapon" attack, does Attack Finesse apply to all targets or a single target:
In addition, once per turn you can deal 1d8 extra damage with a weapon attack using a one-handed weapon, a garrote, a blowgun, or a shortbow. The extra damage increases to 2d8 at 11th level and 3d8 at 21st level.