rthorne,
Try using programmable mix 5 or 6 instead of programmable mix 3.
I am setting up a UCD 3D with two elevator servos with the second servo plugged into the aux 3 channel on an 8103. I found a little tidbit of info in the manual. It is suggested to use programmable mix 5 or 6 for mixing elevator servos as the trim function is available in the mixed channel on these program mixes only. Set the elevator channel servo up for max throw and then slave the aux 3 channel to it to match throws.
Hope this helps.
James