Yes, the Era 100s will work great as surrounds with the Arc and Sub. Just set up the Era 100s in the Sonos app and add them as surrounds:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

If you wish you can also set the surrounds to “Full Mode” when playing music and they will mirror the front L/R channels.

Thank you for the replies! My understanding from reading more in this community is that the Era 100’s will be great, as long as they’re receiving the music through the TV HDMI-Arc. When trying to play stereo music through the Sonos app, not so good. Do I have that right?                                                      Sorry for all the questions. I just don’t want to spend $400 and be disappointed by some technical snafu.

When you add surrounds, they become part of a Sonos ‘room’ (think zone). When you play music to that ‘room’, there are two modes, where the surrounds support the sound bar (Ambient), or play as full stereo speakers (Full). You also have the options of volume for those surround speakers in comparison to the sound bar, but it isn’t possible to turn off the sound bar, as it’s part of the ‘room’. 
 

I think you’ll find that for either TV sound, or for streaming music, the Sonos solution will sound great. 

Buy direct from Sonos and you’ll probably have a no-quibble home trial period. If you don’t like, just return them. (Check this is available in your country - your profile doesn’t say where you are.)