I was thinking of adding foot-switches to my splitter, but not sure if I want FSs on the mixer as well.
However, to do so for what you have in mind, Randal, totally makes sense.
What's it fit in? Ahh 1590N1. So you'd want to have a 1590BB to accommodate the foot-switches.
Wires from the board to the jacks would be interrupted by the footswitches, with the input grounded when bypassed.
You could try cramming in some Smart relays if you wanted to be able to radio-button selections, ie with the banjo on when selecting bass the banjo automatically turns off — great if you're manning all three instruments, but not particularly useful if your band-mates are involved and all need to be mixed in at the same time.
It's a tight squeeze for 3 footswitches in a 1590BB, but spaced as far apart as possible and staggering the middle one, it should prove accurate, even if you have big feet like mine. Something like this: