I just bought a house and I see in the garage there is a subpanel with only 3 wires ran from the house. There is no separate ground wire and I understand that using the ground and neutral on the same wire in a subpanel is a bad idea. I was wondering if what the best temporary fix for this since I can't get a new wire pulled right away. Would it be safer to pound a ground rod out by the sub panel and isolate my grounds and neutrals until I get a wire trenched in.

Driving a ground rod and grounding both the neutral and the equipment ground together with #6 copper wire is allowed, for now. This method will not be allowed in the next code edition.

A fourth wire is best, but the rod will suffice.