When you play Falling Star targeting 2 of your opponent's units, and your opponent bounces one of those units in response, how much damage is dealt and to which units?

You deal 3 damage total to the remaining legal target. When your opponent bounces a unit in response, that unit becomes an illegal target when Falling Star resolves (because reflexive triggers don't select targets until the spell resolves, after responses). The bounced unit takes 0 damage, the other unit takes 3 damage, and the second reflexive trigger has no legal target. You would never be forced to target your own units.