You can directly use a real A-510-x Starter/Magneto Key for your cockpit. It's full 5-position (OFF-R-L-BOTH-START) switch, but unlike the usual 5-postion rotary switch now you can use it with 3 inputs only.
Connect three L,R,S terminals to the controller board or multiplexer board inputs as is shown on the picture, the GRD and BAT terminals connect together and to the GND:
In the configurator select the related parameter in the [Ignition/Start] layout map (A-510 Starter/Magneto Key).
This custom function are added to SimVim for separate magneto switches for single-engine planes that have two separate toggle switches for L and R Magnetos instead of rotary key switch.