Remove (unbond) the Era 100s from the Arc as surrounds and unplug all three speakers from power and reboot your router and phone. Swap the Era 100s (left speaker is now right speaker, right speaker is now left). Plug everything back in and try to add the Era 100s as surrounds again. Also be sure to keep WiFi enabled on all speakers in the Sonos app.

Thanks for your help…  I was able to fix my problem, but I used a different approach… I factory reset all three components, the arc and both era 100s.  I then plugged in the era 100 that I couldn’t get connected first, the app found the speaker and I was able to get it configured and updated and added to my system. I then followed the recommended process for the second era speaker and finally the arc sound bar. There were some unexplainable glitches where I had to reconfigure the WiFi after adding each component but all three speakers are now connected tuned and configured as a surround sound system