KD4UPL wrote:
To meet code you need to drive a rod at the garage and ground the panel.
LIke a lot of other problems posted here......you are making this a lot harder than it really IS.
The 3 existing wires have to get into the building through some kind of a hole.....right ?? Drive a ground rod OUTSIDE near that hole and run the ground wire into the building through the same hole that the other wires already use. If necessary, chip out a little mortar or sealant to make room.
If you trace the feed wires back to a pole, there should be a ground rod at that pole, connected up to the transformer. That's where the real ground/neutral makes it's connection.