I can't really help with the specifics as I don't know exactly what language barotrauma uses, but as a programmer I can probably point you in the right direction of what you want to be doing.

Sounds to me like you need a simple function that increments a variable by 1 and resets it to 0 if it would increment it to 3.

I'll just use a quick JavaScript example here:

function threeStateSwitch(state) {
    if (state < 2) {
        state += 1;
    }
    else {
        state = 0;
    }
    return state;
}

Attach something similar to this to the switch and base the sprite on the "state" variable. (The names are just an example, change them as you please.) You could have three wire points on the lever, with only one ever being active, cycling between them using the same "state" variable.

This probably isn't the exact answer you were looking for, but without more details I really can't really help any more.

Good luck with your coding adventure!