You wish to add your Era 300’s as surrounds for the Beam, so they are all in the same room?  See this link:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

Note: If your Era 300s are currently paired, you need to separate them first.

https://support.sonos.com/en-us/article/separate-a-stereo-pair

What makes you need to delete a room? Generally speaking, rooms are constructs of speakers. If you rename a room that contains a speaker, the old room name should disappear. Similarly, if you move a speaker from a room (like when adding surrounds, they start in one room, then get bonded to another) the room should disappear. 
 

The thing I’m semi confused about in your post is if the Era 300s are ‘bonded’ as surrounds to the Beam, or are they set up as a stereo pair in another Sonos ‘room’.

If the former, there is a setting in the Sonos controller, in the area for the Beam’s room to set playback for music as ‘Full’ rather than ‘Ambient’. I highly encourage this, I’m not sure why Sonos chooses the default to be ‘Ambient’. You can, in that same area, adjust the relative volepume of the surround speakers to the Beam.

If it’s the latter, all you have to do is group the two rooms together. 

A warning, the Factory Reset usually does more harm than good and can be difficult to recover from as well as erasing any internal log data Sonos could access to assist you.

Like the FAQ suggests, you’d usually save that for after you consult Support.