I have the Pixhawk 3 PRO system without the all in one module. When I use shorter USB cables to connect the system to the laptop, it turns on and boots up. If I am using some other longer cables, it does not turn on.
I have checked my cables and they are working fine. I can even use the same cables to connect to the Pixhawk 1 autopilot without any problems. However the Pixhawk 3 does not work for some reason with these cables.
For my purposes it is very important to have the ability to connect to the groundstation via USB when there is no external battery power available. I was able to do so with Pixhawk 1 but unfortunately not with the Pixhawk 3 PRO.