If a source would deal damage to multiple applicable recipients, the prevention effects apply to each of those recipients separately. For example, if Hedron-Field Purists is level 1 and you cast Earthquake (“Earthquake deals X damage to each creature without flying and each player”) with X equal to 3, that Earthquake will deal 2 damage to you, 2 damage to each creature without flying you control, 3 damage to each other player, and 3 damage to each creature without flying other players control.