"If the balance of the force is with the dark side during the dark side players' balance phase, the dark side players may deal 1 damage to 1 vulnerable light side objective."

Is that 1 damage no matter how many DS players there are, or is it 1 damage to 1 vulnerable light side objective per dark side player? 3 DS players, 3 damage.

1 damage.

In multiplayer, there are not 3 Balance phases, but 1 phase shared by all players.