Accessory0 thru 3 are set up when you do the transmitter wizard. They can be mapped to outputs on the Vehicle page in the lower right corner. That would give you 4 direct channels from transmitter to servos.
A way to get all channels available would be to install a PPM to PWM converter (a second CC3D could even be used to get a different set of channels from PPM to PWM. Another way would be to install a PWM receiver in parallel with the PPM receiver.
From the FC you get a different 4 "stabilized" channels to control the standard functions of roll, pitch, yaw and throttle. Also a fifth channel for the flight mode switch controls FC flight modes.
You only get these 4 (orthogonal) functions output from the FC.
I am guessing that you only need two balancing functions, call them roll and pitch. You probably want to mix these two outputs into some group of your servos and have others controlled directly by the transmitter. You can set up a custom mixer to send various mixes of these functions to more than 2 servos.
You would need to explain how each of the servos is used and what is needed (for balancing?). Then someone with some experience might chime in, but around here we know more about quads and planes than robots.
