how to program a 3 positions switch for one

If you want a switch to have the values -100, 0, 100:

complex-mixer (3 mixers):
mixer1: src = SW0, switch =sw0, curve=fixed, scale = -100, mux = replace
mixer2: src = SW1, switch =sw1, curve=fixed, scale = 0, mux = replace
mixer3: src = SW2, switch =sw2, curve=fixed, scale = 100, mux = replace
(Make sure to set the src and switch before setting the curve. If the curve is already 'Fixed' change it to 1:1, then set the src and switch, then set the curve back to 'Fixed')