why are you linking or mixing it? your third channel should be seperate. you only use the third channel when you shift into reverse, so if you mix it with another channel it is not seperate.
there should be a switch or lever or button that activates your third channel.
unmix it. It sounds to me like when you hit the brakes it shifts, which is what it would do if you mixed it with channel 2.
you created your own problem!
