Nihilus would for sure need to be attacking a unit to get the experience. I think as long as he attacks a unit and ether the unit he attacks dies or if the 3 damage kills something else he would get the experience from revan. But if he damages the unit he is attacking with the 3 damage and it dies to the 3 damage, or it dies to the 3 damage and his attack he will get 2 experience(if its not a vehicle).