Dual Rudders
I have 2 8103's and I have always used channels 2 & 6 for ailerons, 3 & 7 for elevator and 4 & 8 for dual rudder, if I have them. I would use mixes 5 & 6 on elevators and rudders as they include trim.
I admit I have never used all 3 on the same plane, but I see no reason why they won't work. You have enough channels and mixes for el & rud and the ailerons are done when you set flaperon wing type.