I assume you want to conner your transmitter 3 way switch, i.e. connecting the corresponding receiver channel to board (PWM)
You can connect to any of the FC_ROLL, FC_PITCH, RX_ROLL or RX_PITCH. Just assign on the GUI accordingly.
So if you connect to RX_ROLL, then on GUI RC input mapping select CMD = RC_ROLL_PWM ( I assume, if that does not work try EXT_ROLL_PWM, sad that the GUI and board labelling is not consistent)
After set the 3 actions under Command assignment as you like.
Black servo wire goes to GND and red to +5V and white to the unlabelled pin
I think he wants to switch the profile using a 3 way switch. The usual way we do that is with what Garug mentioned. Using a RC transmitter with receiver.
What the CMD detect is the voltage so a switch will not work. You can try a rheostat switch.