two era 300 and a sonos beam

You can get a very good 5.1 setup adding a bar. In my opinion, Era 300 are absolutely underused as surrounds, but the result is awesome.

As Era 300 are the best, I would try with an Arc Ultra, but if the budget is limited, Beam is also a good option and Atmos capable.

I don’t know how this Raku device works, but if you’re streaming the audio through a Bluetooth connection you’re exploring just the 40% of the Eras’ pair. Try using Sonos app and Amazon music to play tracks produced in Dolby Atmos, it’s great.

And don’t worry, once you get the whole setup you don’t need to unbound Eras speakers to listen music, I think it sounds much better with the bar included (and if there is a Sub even better 😅)

I guess I am missing something, why aren't you streaming directly with the 300s?

I stream directly to my 300s and they do a wonderful job reproducing Atmos content. 

Sorry, i didnt make it clear. I stream audio through the Sonos's App, but for TV, i can stream the audio via bluetooth for the pair using a Roku App. 
My question is, if i add a Beam, will the Era 300 be underworked?
Will atmos content be reproduced within the three devices? 

With a Beam 2 and Era 300s connected as surrounds, you will be able to get Atmos content from both TV and audio sources.

Bluetooth is stereo, not a 5.1 signal. 

Bluetooth audio quality is often horrid, on top of all the other issues. It is a last resort connection for me.

If you’re sending the TV sound through Bluetooth to Eras, you should suffer some kind of delay.

Anyway, with Beam you will connect TV directly by HDMI and the Eras will sound perfectly because the bar manage the sound in a quicker and wider way than Bluetooth.

And yes, as surrounds, the Era 300 are not the main characters, because the Beam takes control. But with them you’ll get an awesome result. Yes, if the content you’re playing is Dolby Atmos Beam and Eras will put sounds upwards.