For a normal 3 way switch, it only sends signals at up or down positions. In DCS you should look for bindings sometimes called special for joysticks like "Up action A else action B", "Down action C else action B".
Unfortunately there is no standard binding structure in DCS and it tends to vary from module to module, official and third parties. If you can't find the binding the suits your switches, you can always try wishlisting them.