I think you cant disable the forth state.
In the hardware world it work slightly different I think. You would connect a 3 way toggle. From guitar pickup select switches I know these work like:
0 = Signal 1 On
1 = Signal 1+2 On
2 = Signal 2 On
To make this work with axoloti you would I think reverse it and feed the toggle the VDD and than make signal 1 2 two pins.
You would then use some logic to turn into something useful: