The only soundbars currently being sold that are compatible with the Play:1 for surrounds are the Ray, Beam Gen 2, and the original Arc.  Arc has recently been replaced, so there’s no telling how long it will remain on sale.  Of those three, I would probably go for the Arc, if you can afford it.  Otherwise, the Beam Gen 2.  The Ray just doesn’t have the features of the other 2 and relies on old connection tech (optical) instead of HDMI-ARC.  Only consider the Ray if your TV is an older model that doesn’t support HDMI-ARC.  

Even looking at the cost I’d be tempted to get the Arc Ultra. The Arc is still available but Sonos has it listed as “Last Chance” in their store.

The Ultra isn’t listed as supporting your Play 1 or 3 speakers as surrounds so you’d have to add them at some point. I’d lean to Era speakers, which depends on your budget and space.

https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations?product=arc-ultra

Your Sub should be good.

https://support.sonos.com/en-us/article/set-up-your-sonos-sub

Thank you all. Really.

Cost is a constraint, I want to get the best possible result for less money.
But reading around, I had another doubt: if I choose the beam gen 2, can I use the surround system to stream music with the TV switched off? I'm pretty sure I can do this with the original Arc, but not with the Beam.
Does anyone know? 

Yes, Amazon Music, Pandora etc or radio stations via RadioPlayer, MyTuner etc (subject to any geographical territory restrictions) can all be streamed to any Sonos Playbar. TV does not need to be switched on. 

Don't even need a TV, my Beam gen1 lives on a shelf in the bathroom.

I did it, I just bought arc soundbar, added sub and play:1s. thanks so much all.