I think your ambition of 7.1 will be thwarted, the Sonos system you have won’t support that. The best you can do with that list of speakers is 5.0, if you added a Sub, you could go to 5.1.
You can use a pair of like speakers to bond with your PLAYBAR to act as surrounds, or you could connect your CONNECT:AMP to act in that fashion, following the instructions in the setting up Amp or CONNECT:AMP as surround speakers FAQ. You could then use the others in another Sonos ‘room’. Note that there is a slight delay when grouping the TV input with other ‘rooms’, but playing music via a stream will be completely in sync.
You may have a bigger issue, it your PLAY:3 doesn’t stay ‘grouped’. Although if by ‘grouped’ you mean with the PLAYBAR, there is a function you can set to auto-ungroup speakers when switching to the TV input. You may want to call Sonos Support directly to discuss it.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.
