Cust.,who is also a builder I work for, wants a sub panel in detached garage via the 3 wire he alresdy has; should I drive a ground rod there?

Yes, but you should only use the three wire method if there is, and will be, no other metallic paths between the two structures. This includes metal water or gas pipe, telephone or other wires. If you have no other metal connection you should drive a rod or two per requirements, and bond to neutral like service equipment.

Great advice from Watthead, the code section that you need to read would be 250.32(B)(2)

Also no matter if you run a separate EGC or not out to a detached building you must connect grounding electrodes to the panel grounding bar.

Bob

s'what I thought - many thanks