The black and red from the receiver to the servo is the servos power supply, usually 5v to 6v which is always on, Its the white wire that sends the signal for the servo position.

I use 4 micro switches positioned at each end of the servo arm a pair for left and right(turn signals), one for reverse and one for front flashers (looks like running lights.) No need for a third servo.

Just to give you an idea.