Grouping 3 Speakers...

@WickedDNA

First, to speak the same „Sonos language, let’s talk about the terms.

There are „physical rooms“ like your living room or kitchen. 
Then there are the „Sonos rooms“ far in the past also called „zones“. 
A Sonos room is a certain configuration of devices. That can be a single speaker, a stereo setup or a home theater surround setup. Each of that also can be extended with a bonded sub. 
The ht surround setup would consist one of the Sonos ht devices plus a pair of bonded (not grouped) surround (rear) speakers. 

If you like, you can individually put two or more of your „rooms“ (configuratios) together as a „group“. 
Volume level can be adjusted for each room of a group separately or for all rooms together. 
 

All that said… the first fact regarding to your setup is: It’s not a surround setup as there is no ht device and bonded rearspeakers. What you’ve got is a „group“ of a single speaker room and a stereo setup room. 
So the audio signal on each room is stereo and not any kind of surround (multichannel) signal. 

And as described above you can adjust volume of a group. 
Tap the main volume slider to get access to the slider for each grouped room and adjust them separately as you prefer. Once set you can use the main group volume slider and it will affect all rooms proportionally. 
 

Sorry for the extended explanation, but imo it’s essential to understand how the Sonos system works. 😉

Okay, I have 3 sonos speakers in the same zone..

Play 3 - essentially as a front speaker

Sonos One SL - side speakers

Sonos One SL

I’ve set up the One’s as a stereo pair and added the Play 3 as a standalone.

It would be preferable to group all three in the same room and not just in the same zone.

The reason for this is volume calibration and distance sat from speakers.

At the moment I have to do it manually and can increase all speakers using the volume button on my phone. 

Small in/decreases are proportionate but larger changes unbalance the output.

Can all three speakers in the zone be bonded(?) to the same room?
 

No, to enable surround sound for watching TV a Sonos soundbar (Beam, Arc, Ray) is a requirement.

As explained above ‘Zone’ and ‘Room’ is both the same thing in the world of Sonos: a ‘Zone’ or ‘Room’ can be a stand-alone speaker, a stereo pair or a bonded home theater setup.

https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations

You can merely group the Play:3 and the ONE stereo pair together to adjust the volume accordingly.

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

In order to listen to Stereo music in 2.1 you would need a Sonos Sub or a Sub Mini (preferably) attached to the ONE stereo pair.

Frankly, I’m surprised you think the Play:3 is adding anything to the quality of sound you are hearing. Sonically it’s quite different to the One SL’s. Just use the Ones as a stereo pair and you’ll probably experience a much wider soundstage and better stereo separation. You certainly don’t have a 2.1 system as you have no Sub to play the deeper notes. And if you have all three speakers playing at once your sound stage will become very muddled.

If you’re using these in some way as a home theatre system, and this is why you think you have a 2.1 system,  how are you linking them to the tv? 

I’ve had a stereo pair of One SL for a few years.
The sound is great and I love them.

When sat in the window with my feet up, I’m between the speakers and while it sounds good, there’s dead air in front of me which I’ve wanted to fill for a while.

My initial idea was a Beam but even used, the cost is prohibitive.

Having added a Three to the bar table has filled the dead air and I’m immersed in sound.

If there’s a better way to solve this problem, I’m all ears.
Once I work out compatibility issues I’ll likely replace the Ones with two more Threes. If I do, can I group all three together as one room?
 

Hi.  IMO, the only thing the Play:3 will achieve here is to muddy the stereo imaging from the Ones.  I would put the 3 in another room.  (Another physical room, that is.  It is already a separate Sonos “room”, rightly and necessarily.)

As others have said, this is not surround sound or “2.1” sound..

But if it works for you, in the acoustics of that room, just leave it as it is and enjoy it.

Having used all three together, the sound is fine.
I’m happy to learn more about the differences though.

Should I not use Threes at all?
I was thinking of picking up more Threes.

Why are you obsessed with 3 speakers as one room? It’s been explained. A stereo pair is the most (other than a Sub as a third speaker) unless it’s a home theatre setup with a Ray, Beam or Arc. Whatever speakers/rooms you have you can group them to play together. You can create and save named room groups too. 

I have nothing 3s in themselves.  If this setup gives you the sound you like, that is fair enough.  I can only talk about how it is likely to sound.  You can hear how it does sound.  So don’t be put off by me or anyone else on here!