I haven't tried this, but don't see why it wouldn't work. You don't need to control all 3 volumes, just match 2 to the other, so in theory a Boss LS-2 could be used?
Bass 1 goes into the Input Jack
Basses 2 and 3 go into the A and B Return jacks, set the volumes of each to match the first bass.
Granted you would have to change the LS-2 setting to access all 3 basses, for example setting it to A<->B would switch between basses 2 and 3, but then with a twist of the Mode knob you could set it from A<->Bypass, giving you bass 1 when bypassed and bass 2 when engaged?