I guess I am confused as well, I am assuming your forwarding path looks like this:
Phone 2
External Caller --> # Assigned by PhonePower --> Google Voice ----|---- Phone 1
Phone 3
In this case, forwarding from GV to Phone 1 / 2 / 3 matters, especially what providers these phones are using and the number the external caller is using. If PhonePower messes with the incoming caller ID value, then that may also be causing some issues as well (GV does not like incoming calls with CID set to itself, since it knows the call setup in this manner is not possible and many times forwarding will not work).