If you want three functions, you'll need more than one binary switch. If I'm thinking correctly, you want the following functions:
1. Amp one activated
2. Amp two activated
3. Tuner activated
This would need two switches, one to toggle between amps, and one to activate the tuner. You could use a fun dual color led to show which amp is activated!
If this is what you want, I can draw up a layout for you.