Though the Era’s can be used the way you are using them now, Sonos’ way is to use a soundbar for fronts and Era’s as rears.

Thank you 106rallye!

Do you know whether my existing equipment (3x ERA 300 + sub) is supported by the soundbar? If so, I would prefer this solution. The sound with my current setup is just terrible, I wasn't expecting that.

The best signal Sonos can receive via Bluetooth is stereo, so in using that method, you’re loosing any semblance of a 5.1 signal. As suggested by @106rallye , your best bet would be to add a Sonos soundbar, which handle the higher bandwidth signal, and contains the front three channels. You could then add two of your Sonos Era 300s as surround speakers, and the Sub to the soundbar. The extra 300 I would just use in another ‘room’, as it can’t be effectively associated with the other speakers…it can be ‘grouped’, but there would be a delay of 75ms minimum between the two rooms. 

hi

Great advice by @Airgetlam and @106rallye.

Just to add for future reference...it appeared that you were trying to use the Era 300’s as left/center/right speakers (with sub) across the front much in the same way of a soundbar or traditional 3.1 home theater setup. That’s not possible with Sonos speakers of any type excepting a Sonos Arc, Beam or Ray. No worries...you're not the first to try and do so; and trust me ...you won’t be the last .

Also, the sub is only capable of receiving the low-end frequencies from either a single Era 300 or two Era 300’s in stereo pair. So in your situation either one or two of the Era 300’s was not working in conjunction with your sub.

Hello everyone,
Many thanks for all the well-founded information. I should have contacted the forum BEFORE buying. But at the time of purchase I was not aware of the forum or the problems I am now facing .
OK, it comes down to a soundbar. I'll have to look into the connection options. I suspect it will have to be a SOUNDBAR WIRELESS SONOS ARC. @Airgetlam: I would really like to have the third ERA300 in the background, i.e. behind the seating position in front of the TV. Do you think the 75 ms will be an audible problem?
Thanks again to all “co-thinkers”, regards Opus

You cannot connect 3x300s in a surround system 

you can only use 2 as rear surround speakers 

and it’s not just audio delay all 5 channels would go to the odd man out 300 grouped room and frankly sound awful 

Yes, the delay, when watching the TV, would drive me crazy. Of course, there would be no delay between the two Sonos ‘rooms’ when streaming music.

In a former house, I used to group my home theater room with my kitchen, mostly during sporting events, so I wouldn’t miss anything while cooking. As long as I didn’t stand in the doorway, where I could hear both rooms at the same time, it worked perfectly for me. Of course, streaming any music to my system was in full sync.