3 amps is tough. I see this one for two amps into one or two speakers:
https://www.amazon.com/TC-7220-Amplifier-Selector-Comparator-Crossover/dp/B00LNOAGZQ
I would use this between the preamp and amps (unfortunately, no longer made but can find used):
https://www.amazon.com/Niles-Black-FG00002-Auxiliary-Selector/dp/B00006I5J0
https://www.amazon.com/TC-7220-Amplifier-Selector-Comparator-Crossover/dp/B00LNOAGZQ
I would use this between the preamp and amps (unfortunately, no longer made but can find used):
https://www.amazon.com/Niles-Black-FG00002-Auxiliary-Selector/dp/B00006I5J0