Barry, you should be able to do this using the Gyro function in the DX18. In the System Menu, Aircraft Type, after the wing and tail type have been selected, press NEXT and set Gyro 1: to Active. You should then be able to go to the main menu and select Gyro 1. In this menu you will see Inh > Inh, change these to be the desired switch and the gyro channel I'll assume switch D and X+8 so set them to D > X+8. Just toggle the switch to select D for the first field. Scroll down to the Switch field at the bottom of the screen and set this to D as well. This will select the curve to be used for each switch position. Set switch D to the desired position to output 35% and scroll to the blinking field in the curve values fields and change it until you see 100 -> 35 at the bottom of the screen. Repeat this process for each of the 3 switch positions and values and you should be done..

Hope this helps.

Edit: Added attachements with SPM files to implement 3 pos switch with positive values for each position
File 3PosSw uses Dx18 Gyro function to select values with Switch D and output them to X+8.
File 3PosSwDS uses the Dx18 Digital Switch function to select values with switch B and output them to X+8. It also
uses the Gyro function to output Switch D to X+7.