Power Selecting between 3 power sources
You could use an old-school relay switching network like this...







simulate this circuit &ndash; Schematic created using CircuitLab

This is just an outline of the circuit. If you implement it using real relays, you would probably want to put a fly-back diode in series with each relay coil , although some relays have these built in. 

You could also implement the same idea using solid state components like transistors, resistors (wired as an inverter) and normal MOFSETs in place of the relays.