lets say i have a full naked unit of rebel troopers, two of the in LOS of the attacker and 2 of them LOS is completely blocked from the attacker. If that unit is dealt 3 damage, the first 2 will go to the visible ones then where would the 3rd one go? RFG says that minis with the LOS completely blocked cannot suffer damage

If none of the attacking minis can see a mini from the defender, this mini cannot suffer wounds at all from that attack. It doesn't matter if there are still 1 or 10 wounds to apply.