Make sure the Era 300’s are not a stereo pair. If so break them as a stereo pair and they should each show as separate rooms. Then go to the room with the Arc Ultra and select “Add surrounds”.
Speakers will be part of a ‘Room’. If two speakers are ‘Bonded’ as a stereo pair, they become a single Room. Likewise, if speakers are Bonded as a surround package, they all become part of that Room. This makes sense because it is much easier to manage a Room (music selection, Volume, Tone, etc.) than a gaggle of individual players.
Try streaming music to the room, and you should hear it from the eras as well as the Arc. If nothing when then watching tv, you may need to adjust some tv settings.
To get more music from them when streaming music set the Surround Mode to Full instead of Ambient.
Once that is working sort out the TV issues.