I’m guessing you have a managed switch involved, which likely means a network storm, due to improper configuration. 

Oops. Forgot to paste the link:

https://support.sonos.com/en-us/article/using-sonos-with-a-managed-switch

FWIW - It’s a Netgear router. Thank you very much for the quick reply. I’ll read that.

Hmm. I don’t think that applies to.

My router is more basic (perhaps too much so?) 

NETGEAR Nighthawk X4S C7800

Without delving too deeply into the router specs, why do you feel the need to wire all three Amps?

You have no other switches or network devices?

Same problem occurred when it was wired. I thought a wired connection would be preferred. Less of a storm - as you say. 

No other switches. Other devices include TV, 2 macs, 2 iphones.

I suspect the router is old, not up to the task. Switching them out is not hard, just want to make sure it’s that versus … 2 sonos Amps with never work together.

I mispoke above. Wired, Wireless = same result. I tried all combinations of wired and wireless. No joy.

I use a R6400, although I don’t have multiple amps, and I’ve run both wired (single device) and unwired, without issue. I suspect there’s still something in the settings of the Netgear. 

That is very, very helpful. Thank you for that. It greatly narrows my focus. Can you tell me where you looked for proper configuration? What documentation did you use or was it all out-of-the-box and bam. It worked.

For me, it just worked. Search the FAQs in the Sonos DB, or call NetGear’s CS. Or look through that link, I wonder if something shows up in your router’s settings.