High quality Speaker switch box for multiple amps?

This is a link to Van Alstine comparator as bugredmachine recommended:
http://avahifi.com/products/accessories/abx-comparator-switchbox
And it safe to switching bridged Amp.

To make those Amplifier switcher safer for all Amplifiers, parallel all speaker outputs with 25 -100 ohm  5watts resistor.

if testing, you could also try running one amp into each speaker and using the pre-amp to "switch"

this won't test the sound staging and some other things, but costs nothing

Regarding a line-level switchbox to go between your preamp and the two or three power amps, and assuming that all of the connections involved are single-ended (i.e., RCAs), I’ve had good results with the DB Systems model DBP-2JAU/5 ($110) listed near the top of this page. I've used it in a couple of different 2-in / 1-out applications, but it can also be used 1-in / 3-out among other ways. It is similar to the DBP-2J/5 for which a review and description are linked to, except that it has gold-plated jacks. For contact information go to their home page.

Another possibility, although it costs $359 plus options, is the Decware Zen Switchbox.

Good luck.  Regards,
-- Al

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