GPU and APU
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.
When were running APU we usually run both packs until people are off. Then we shut the packs down as well as the lower recirc fan. This is if ground personnel later hook up air. Our company has had to replace a couple lower recirc fans getting worn our because of the air pushing against it.
We also shut down the battery (unlike other Boeings the APU won't shut down). We do this to save battery if the APU were to shut down or External power is lost at the gate. Otherwise you end up replacing a battery that runs somewhere between 70K and 100K.
I have never run just one CAC. If were on ground and people are coming aboard when I crank the APU I turn both pack on and lower recirc and call them and tell them to unhook ground air. Ground air is usually not so good on bigger jets. It will cool and empty airplane fine and keep it reasonable but as soon as you start filling it up doesn't do the job.
Ken M
787