Option 3 as you cannot add just one Era 300 as a surround. You need two speakers of the same type to add as surrounds.

I thought the whole idea is that Era 300 can produce an holistic sound to all directions itself..

Will it not create a good enough effect if stands alone in front of Arc and Sub?

Thanks!

As I said, you cannot use just one speaker as a surround. If you only had one Era 300, you could only group that with your Arc/Sub, but that would replicate the audio (in stereo) from the Arc with a 75ms delay, which is standard for all grouped speakers when playing tv audio.

To have the dedicated rear channels, you need two separate speakers.

I see, OK thanks. 
Do you think of another feasible option of placement here?

Maybe double speakers of other model/brand?

Not sure if double Era 300 is needed in our space or a total overkill..

Thanks!

Double Era100? Double One SL? 

Surrounds are for directional as well as ambient sounds.  You need two of them for directional, so sounds can go left to right as well as front to back.  Era 300s are also Atmos capable, so sounds can go up and down and over you head. 

Since you're very close to the rears and the 300's should also be placed differently than you have any possibility for (preferably at ear height and, farther apart while keeping distance from the side wall), I'd go with two Era 100's.

That’s interesting, where would you place 2 Era 100’s then? What are their placement requirements?

Thanks to all repliers. 

Given the proximity to the rear and the specific placement preferences for the 300's, it would be ideal to position them differently. Opting for two Era 100's could be a suitable choice, especially if they can be placed at ear height and slightly farther apart, all while maintaining the required distance from the side wall.

The wall is right behind the sofa so I want be able to clear space behind the Era 100’s. 
Is it fine having them close to the wall behind them but far from our heads on the sofa? 

I agree about placement and height comments here. Makes all the difference. I will say hanging the 300’s on the wall required care due to their weight and size.

I have ARC, Sub, and 2 Eros 300’s for surround. I made the mistake of buying the 100’s as well. The 100’s don’t work in the home theater config. I wound up using them in another room. The latest addition is another sub placed diagonally from the other. This ultimately wound up being the optimal config. True play the whole setup and it really kicks.

Finally, while I’m now happy w my SONOS setup, it would’ve been nice for SONOS to make it easier and more straightforward (since that’s supposed to be their thing). If I had it do all over again, I’d give much more serious consideration to BOSE Ultra home theater w surrounds.