Change Channel for throlltle

I have a pixhawk and an APM with ardurover for baitboats, so only need Steer ( CH 1 ) and Throttle ( CH 3 ).
To use a particular software, I have to change Throttle Channel 3 to Throttle Channel 2.
So, on Output Channel in MP, I have disable CH3, define Throttle on CH2, calibrate radio again, and change the ESC to position 2.
The problem is in MP, CH2 neutral is staying at the bottom and so Throttle is always working…
An idea ??
Thanks a lot for your help.

Thanks a lot to the community for your help !