Put the spare One on the BBar and then group them.  You can even use the new Zones feature, but you’d have to decouple the stereo pair to do it.

Okay, I looked at zones, so if I do this, how do I automate volume increase/decrease while maintaining correct balance?
Each speaker will be slightly different distances from the bbar.

Plus, wrt the sound experience.
What stereo setup will each speaker have?
With my preferred triangular setup, I’d have one front speaker, hopefully outputting in… what? How does that work? ..then a L&R speaker in addition.

Forgive me, from previous posts, I know there’s preferred nomenclature and I don’t want the question to get bogged down by terminology.

In a Zone, each speaker is mono, playing both channels as one.  If you group the two (stereo pair and BBar), the stereo pair will play in stereo, the third one in mono. 

For roughly the cost of Beam2 you could get an Era 300 which is designed to create a good spread of sound. 
Its design should create a talking-point for your guests too

Makes sense, but I pick up used Play 3 for £65.
 

Yeah, that’s the plan for the moment.

The thread is a feature request, so hopefully Sonos can address small problems and increase CSAT.

The thread title is asking for grouping speakers. Grouping speakers is already a supported feature. 

If you are attracted by the PLAY:3 £65 cost, install a stereo pair on the window ledge. Stereo listening will be optimal when your listening position and the two speakers form an equal lateral triangle. However, I can’t be sure about the dimensions of your space. Window ledge L/R speakers may be a little close to the table for optimal L/R because one chair will always be too close on either left or right.

You can still Group stereo or mono speakers on the other side of the room with window ledge speakers. Sometimes a mono Grouping or Zone of speakers is more satisfying than a haphazard gaggle of stereo and mono arrangement.

I’ve always had difficulty placing a PLAY:3 on a window ledge, due to the way the power cord comes straight out of the back. It’s not huge, but I’ve always preferred the PLAY:1 or Sonos One, both of which sit on their power cords for that type of location. Putting my PLAY:3s on speaker stands helped resolve that issue. 

I have not investigated any of the details, but is may be possible to find a replacement right angle power cord.

I was thinking the same thing, back in the day…or a 90 degree adapter, which I’ve used on HDMI in several previous cases. But it really isn’t an issue at this point, as I purchased speaker stands for them. Which are probably hard to find these days, since the PLAY:3 stopped being manufactured years and years ago. 

Does it work the way I want to though.
Just for music with one, effective, front speaker and two sides?
That’s what I want to achieve.

Nothing really “works” playing a stereo signal through three speakers.  Stereo is meant for two speakers, if one is looking for the most cohesive “pure” stereo sound.  Once you add the third speaker, stereo goes out the window, and mono vs stereo pair + third speaker really doesn’t matter.  

What do you mean by “one, effective front speaker…”? 

You can set up a “room” with one speaker and a second “room” with a stereo pair. Then you can position them wherever takes your fancy in the physical room where you want them located. You can then set the two rooms in a group, and control the volume of the speakers in their configured rooms, either as a whole group or as individual rooms if you want to change the volume of one room relative to the other. So yes, I think the existing grouping capabilities addresses your currently-stated requirements. That could be done with Beam or other soundbar, Eras, Play:3’s, Ones, etc. For streamed music they will play in sync, whichever speakers you own and group.