What if two cars backing out of a parking spot and a vehicle coming from your right backs out the two cars collide right rear bumper to right rear bumper who is at fault?

Both partys are At Fault and each party will need to file there vehicle under there own insurance. Parking lot accidents are always share fault unless one of the vehicle was parked, but since both vehicles were backing out at the same time, both partys are at fault unless you get a kind person to admit it was there fault.