Best surround placement in not ideal room.

Of your choices #3, but I'd really lean to Era-300s in the #3 location but mounted inverted.

I like 5.1 sound or front Atmos and 100 surrounds (it is what I have since I had no spot for the 300s) but after a bit of listening your really going to find it less satisfying compared to a full front and rear Atmos setup.

thanks for the quick answer :) 

The WAF may limit my 100/300 decision. 

What do you mean by ‘inverted’?  Mounted on the ceiling angled down towards the sofa?

From the Era-300 user guide. The mounting instructions for using the Sonos Era-300 wall mount are in there.

Inverted wall mounting (upside down) Mounting the speaker upside down is recommended if the speaker is installed within 2 feet of a ceiling to optimize downfiring sound. The speaker can swivel left and right, or tilt up and down. When wall mounted upside down, the power cable can be routed from above or below.

I agree that the Era 300’s are the best choice. However you have room to drop them lower on the wall and forego inverting them (mounting upside down). The greenery (in the corner) would have to moved to some degree.  Also it looks like the speaker in that corner mounted high or low is going to be slightly cramped for clearance on the “right” side (when facing the speaker). Although I really can’t determine clearance looking at the photo.

The wall in the corner is 20” wide… 9’ ceilings.  The other side is 18” but opens to the kitchen.

Based on your measurements Clearances should not be an issue 👍

One more question… is the seating center of the sliding door?  If so perfect!

Yes, it is 😎

For an actual surround experience, I'd choose #3 (Era 100s wall-mounted) if you can place them slightly behind and to the sides of the main seating position. Ceiling speakers work well for Atmos but are rarely ideal as surrounds, and #2 can work but often ends up too low and too close to listeners. With an Arc Ultra and Sub 4, properly positioned Era 100s on the wall will give you the most convincing rear soundstage.

I agree that #3 is best. Perhaps mounted under the lights, avoiding the corner wall as much as possible. #1 is worst because one speaker will tend to dominate, due to nearness, depending on exactly where each listener is seated. A listener seated exactly at the adjusted optimal spot could be happy, others may not enjoy the result.

Just one more point if it wasn’t made clear….

Era 100’s don’t process Dolby Atmos in any use scenario. They are stereo only.