Arc, Sub Gen 3 and Era 300

I think your use of the terms is confusing, but in order to use the Sonos Era 300 as a surround speaker for the Arc, you’d need a second one. Surround speakers are always a pair of speakers, never just one.
 

A Sub, or Sub Mini, would flesh out the sound, by reproducing the .1 part of either a Dolby Digital or Dolby Atmos signal. 
 

You can send Atmos to the Arc already, it will deal with the signal without surrounds and Sub as it stands right now. 

What does the Sonos app say you are receiving when you miss surround?

For TV viewing it is not necessary to use eArc to get Dolby Atmos. eArc is need for uncompressed Atmos, but most if not all streaming services use compressed Atmos form a Dolby Digital stream - that can be passed through HDMI-ARC.

Sonos could make it so 1 era 300 as a single rear surround solution but chose not to  like pairing two arc’s together as surround is also impossible limited by corporate interests i believe 

I do not think there are many suppliers using single rear surround speakers. If Sonos would allow this, they would lose sales (people would buy less speakers) and custumers would get sub opimal surround (probably losing Sonos even more sales).

So what you are saying is that 1 era 300 is 100% capable of playing Atmos music with full surround by itself similar to an arc but no way It could work behind you as part of a group solo what am I missing?I understand speaker separation the wider the better but ..beating a dead horse here, Sonos sounds great just miffed Sonos released Atmos rears a month after I bought 2 sl ones for my arc sub 3 combo and 2 surrounds cost almost as much as an arc or a sub 3. maybe I can use the sl ones as expensive computer speakers for music production.

Years ago Dolby Pro Logic was a four channel analog surround system — Left, Center, Right, and surround. Even though two speakers were usually attached in the rear, it was a mono, reduced bandwidth channel.

In the modern digital era, broadcasts and movies use stereo in the rear. Certainly, the playback equipment could combine the stereo rear channel information and present it to a single speaker, but this is not the expected behavior for the wider audience and this behavior would involve extra work for equipment manufacturers. (Design, testing, maintenance)

In controlled listening, test environments you only needed two speakers or a headset for reproduction of full 3D sound. Soundbars attempt to accomplish this at home, but there are too many setup variations to make this wildly successful.

The rear Era 300s would be receiving height sound cues along with the height speakers in the Arc.  You would essentially have a 5.x.4 system, with the ‘.4’ being front and rear upfiring height speakers.  As to Atmos performance, with upfiring speakers, it is highly dependent on your room.  Flat, even ceilings from 8-10’ high, and sitting the proper distance from the front Arc for the sound waves to bounce off the ceiling and hit the viewing area is optimum for sensing the Atmos effects.  I happen to have an optimum room (9’ ceilings, 13 ft away from the screen) and the effect, even without Era 300s, is dramatic.  

Thanks. My ceiling height is fine, The room itself is pretty small though. Typically sat around 7ft from the Arc with a wall directly behind me and the surrounds 3ft to either side of me. Very recently moved into a bigger place so maybe see better results once I am kitted out.

Hmm.