Several things to discuss.

First, what is currently powering these three speakers? It may be as simple (albeit costly) as adding a Sonos Port to whatever device is currently powering them. Or, more judiciously (three could be an issue, a Sonos Amp, which has both an Amplifier and a Port built in.

Oh. And I just went back and reread your post. I’m assuming your 5.1 system isn’t a Sonos system. Which makes the above, while still accurate, more challenging. A Sonos Port would certainly feed your current 5.1 system a stereo signal, which defines that as a single ‘room’, in Sonos terms. The second device (potentially) discussed above, would be ‘grouped’ with the first, and give you synchronized playback across all devices.
 

Note, however, that all analog line ins on a Sonos requires a 75 ms delay, so if you’re feeding something to Sonos from that amplifier, all connected Sonos devices would be slightly delayed from the speakers connected to your 5.1 amplifier. 

Just to be super clear, any signal from Sonos to your ‘grouped’ devices and the other amplifier would be in sync. Meaning all speakers would be. It’s only when sending a source from the amplifier to the Sonos is where you’d hear the delay. 

That is an odd setup, two amplifiers. Are you sure it is a Connect Amp and not a plain Connect with no Amp or speaker connectors?

Are you running throughput of your 5.1 reciever to one set of voice-coils and the Connect Amp's output to the other voice-coil?

I'm very confused, not sure what you have now and how it is connecfed.

Each of those two channel speakers typically sums left and right from an amplifier, resulting in mono output from that speaker.

If you want stereo in the kitchen, you would connect left or right to each speaker. This is tricky with three of these dual speakers. Maybe one speaker should use both left and right. Let’s say that the speakers are arranged in a straight line A-B-C and your typical listening position is centered on B. In this case connect left to A, right to C and both left and right to B. The best solution might be to ignore one speaker and connect left or right to each of the remaining speakers. In the A-B-C arrangement consider adding an inline speaker Volume control for each speaker.

What was the designer thinking when they decided to install three (3) speakers (dual channel or not) instead of the normal two or four. Three (3) is a very odd configuration. Additionally, with what component were they planning to drive the three (3) speakers with?

I’m aware of a long skinny kitchen with a low ceiling. It’s a food prep area with no room for a table. Three mono speakers would be reasonable. Attempting stereo would result in very small balanced areas between speakers. At all other listener locations left or right will dominate.