You can group a One SL to a stereo pair of Ones but it will act as a separate zone.

Thank you for your reply. I want the three Sonos to work together, so, as I understand your answer, I need a Sonos One (and not SL), so they can make the sound all around the room ?

A stereo pair of any speakers is one “Room” in Sonos parlance.  You cannot add anything more to that Room, except a subwoofer.  You can set up another “Room” and then group that room with the stereo pair.  Unlike paring, grouping is a non-permanent, on-the-fly method of synching two rooms to play the same music.  See this link for instructions on grouping:

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

No, you can get a One SL and GROUP it with the pair of Ones so all three speakers will play the same audio at the same time. Read more about how to group Sonos speakers here:

https://support.sonos.com/en-us/article/group-and-ungroup-rooms?language=en_US

Thank you for your reply jgatie. Then, as I understand, I won’t have a repartition of the sound with the three speakers, but one stereo pair and the third speaker playing the same music in his corner…  So what I want to do seems impossible :-)

You will have two speakers in stereo playing the R/L channels individually, and one playing mono (both R/L channels from the same speaker).  

I don’t understand why having two Sonos ‘rooms’, all playing the same thing, is not what you want. You’d have a stereo pair, and a third Sonos speaker, all playing the same thing, in sync. Where you physically place the speakers is up to you. 

Thank you all for you answers.
Airgetlam, you must be right, and I may be too difficult (or not clear) ; if I place the third one in the middle, it will play L+R , and the two others will play separately R and L.
I was looking for the same sound spatialization with three (because I find that this works really good with a pair) ; my room is in a shape of a L, and I wanted the Sonos app to be able to calculate it, in order to have a real surrounding sound all over the room.
I have, with two speakers only, not enough power (sometimes, I like to play it loud :-)
But, as you all explain, may be it will be enough to add a third one as a separate “room”. 

Perhaps you should consider an Era 300 instead. Take a look at the speaker description and how it will fill a space with sound. 

You can still group it with the One stereo pair. 

But

it may sound somewhat different to your Ones. 
 

Buy direct from Sonos and use the home trial opportunity to try it. If it works, keep it; if not, return it. 

Nik, You’re right, but it’s not the same budget…. ;-)

In the scenario we’re discussing, Room 1 would be left and right channels, going to the respective speakers. Room 2 in the Sonos software would get both left and right channels to the single speaker, doing what I believe you’re looking for.

I’m fascinated that you want this, to me it would be a tad odd, from a soundfield perspective.