G3 will only work if it can detect that a G3 controller is plugged in. If you just have a G2 controller plugged in and try to start G3, it won't load.

If you start G3 with a G3 controller AND a G2 controller plugged in at the same time, then it will load and your G3 controller will have control of the aircraft. You can then use the "Controller" menu to choose which controller should control the aircraft.