control motors connected to the main outputs of a Pixhawk via radio control

Hi everyone,

I am currently working on a design for an airship with two side propellers and one propeller at the rear of the vehicle.
I am using a Pixhawk 2.4.8 as my control board and programming it on QGroundControl
My problem is that when I connect my ESCs or servos to the main outputs of the Pixhawk, I can’t associate them with a channel of the radio control.
I have to connect the motors to the auxiliary outputs and go to “settings” on QGroundControl and type RC_MAP_AUX* to associate an auxiliary output to a channel of the radio control.
In summary: how can I control my motors connected to the main outputs via radio control.