Try this:

http://h1.ripway.com/jaafit/RFG3/Flaperons.plnG3

Here's what I did. I changed the right aileron servo to get input from channel 9 instead of 1. Then I added inputs to channel 9 so that it mixed channels 1 and 8, 8 being the 3-pos switch. Then I added a channel 8 mix to channel 1 for the left servo. The results can be seen in this file if you load the aircraft editor and expand the Software Radio section.