From the sound of it the physical mixer is just going to do what the program does.

I need to get ch3 left to control ch1 forward and ch2 in reverse, and ch3 right to control ch1 in reverse and ch2 forward.

So I think I'm trying to split channel 3 into 2 channels, with each one controlling the 2 in different directions. If that makes sense.

Maybe it would be easier to have each stick control a wheel and then I could turn by having one up and one down. But the left stick doesn't return to centre when released like the right. It sits where you leave it and it's not easy to centre without looking.