Just to be sure. The Era’s would not be “side” speakers to the Arc, but can only be used as rear speakers.

Your question if they would “work with” streaming could be answered with a definite “yes, of course, that’s what they are made for”, but I’d be curious about what your expectations are. The Arc with Era’s as surrounds will doe music, surround and Atmos if your TV and subscription level with TV providers are capable of relaying this to Sonos. So: why this question?

A few things.

1. Sonos doesn’t offer an option for setting up speakers as ‘side’ speakers. If your Era 300s are ‘bonded’ to your Arc (ARC is a method of connection which stands for Audio Return Channel, Arc is a speaker from Sonos), then they’re set up as surround speakers. When the Arc receives a Dolby Digital, DTS, or Atmos signal from the TV set, the ‘bonded’ speakers will be working. If the Arc is receiving a stereo signal from the TV, the ‘bonded’ surrounds will be silent, as there is no channel of information being sent to them. 
    
2. If you’re sending streaming music to your Sonos Arc, then you can go in to your Arc’s room settings, and set up the surround speakers to be ‘full’, rather than ‘ambient’, and they’ll play a full stereo signal.
3. If you’re grouping the pair of Era 300s with the Arc, rather than having them ‘bonded’ as surround speakers, when playing the TV’s input, there will be a small delay of about 75 ms between the two ‘rooms’, but when streaming music to both, they’ll be in sync. T

Here are a few if FAQs that might be helpful:

https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations

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

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

https://support.sonos.com/en-us/article/change-surround-audio-settings

There may be others….

Edit: @106rallye types much faster ;)