So just for your benefit @FILIBAO - when the three (compatible) soundbars are grouped, any Atmos music audio (from Amazon music for example) via the Sonos App, will output in Atmos format to all the ‘grouped’ soundbars - if you were to then group-in a non-Atmos speaker, the audio on all players falls back to stereo.
TV surround sound audio/video will play on the primary soundbar only and display on its connected TV screen - the two other grouped soundbars play in stereo only. There’s a slight delay to the grouped rooms between 75ms and 2s - depending on settings set in the Sonos App - the stereo audio can be set to ‘Full’ output to play on both the ‘grouped’ Soundbars and their Surrounds/Subs.
I hope that assists. 👍
To achieve 5.1 surround sound audio output to all - you would need to look at a wired option using an audio extractor/splitter - the easiest option is a 3-way optical splitter…but note this is just a solution for ‘audio’ output/input only.