If you first connect 2 Era300 to your Arc and Sub, they will act as surround speakers in your setup. They should therefore also be placed correctly in relation to where you watch TV.

You can therefore not only play music on one speaker, the music will come out of your entire system. (Or you can, but you need to remove them from your Arc/Sub every time before playing music)
I play music on my system myself and it sounds fantastic.
(Arc Ultra + Sub gen2 + 2xEra300)

You should be aware that if an Era300 is connected as a surround speaker, the sound will ONLY come from the sides and top of the speaker and not the front speaker.

If you choose not to connect the Era300 to your Arc-Sub, it will act like a regular SONOS speaker that has its own "group/room" in your SONOS App. The sound will also be different as it will play music from all of its speakers, the best setup would be a pair that you then convert to a stereo pair.

Thanks for your reply.

if I connect the 2 era 300, I only have the possibility to place them on the same side of the arc, is it a problem ? Or should it be place on the opposite side ?

thanks again!

It wouldn't work because the sound from the Era300 would be the sound you hear from behind/side or height from behind/side. You would ruin the entire sound image and get a very strange sound experience.

You can compromise and place them as best as possible, but they must be placed opposite your Arc.

Do you have a picture of the room?

I suppose that for the music this is not an issue. Only when we visualise a film. 
see attached the room, the idea is to place them on the white box on the left and right side of the Sonos arc.

what I can do is to move the era when we want to have the home cinema setup by putting them on the sofa?

let me know if you have any idea ?

thanks again!

etienne