'Hey Sonos, group all apeakers'

John’s answer is the easiest, but it’s not Alexa.  You can setup an Alexa group  and put all 3 Sonos rooms as the default speakers for the group.  So if you named the group ‘Home’, you would say “play music on Home”.  If you didn’t want to have to say “Home” then you would need to include the Alexa device in the group so that it knows to use the speakers in the group by default.

If possible I would like for all speakers to play ‘as default’ within the Sonos eco system. 

Well then, as I stated, you can setup an Alexa group with your echo supplied as the voice assistant in the group.  if you aren’t aware of how to setup Alexa groups, here’s a link with info below.

https://support.sonos.com/s/article/3169?language=en_US

Thanks melimbe, I do have an Echo spare so I will try this. I have set up a Home group in Alexa app and that does work.

However, it still seems odd to me that Sonos sells itself as a multi-room system, but you have to use these work arounds rather than setting up a default group that Sonos plays to without any prompt.

I don’t understand your comment at all.  Sonos has an app in which you can group, on-the-fly, any combination of rooms you want, including a one touch button to play everywhere.  That is how it has been since Sonos pioneered multiroom wireless systems (or at least, systems that worked and were user-friendly, before someone mentions LMS).  You can also now group using SVC.

I think Sonos can justify ‘selling itself as a multiroom system’.