We never run 3 GPU's. The book says you can it will help during engine start if you start the right engine but I imagine that would not happen.
Maybe I'm not quite understanding you, however 3 GPUs are a must for Engine start (Right only, system logic wont let you start the left) when APU is inop. Tried it with just two and its not pretty.
The airplane will still load shed with the switch in maintenance enable, the load shed groups will just have different priorities.
You wont do it any harm.
You will when the ground power trips. The CACs are not supposed to be run off just two GPUs, if you want to run the air con without APU then get a 3rd GPU. Simple.
