Cast to Device - bluetooth speaker groups?

I have three Amazon Echo devices, each allows me to connect by bluetooth from my laptop and use the Windows 'Cast to Device' feature to play locally stored music (MP3/FLAC) directly to the speaker.

When I use the devices to stream from online music services, I can play to a pre-set group 'everywhere' and it replicates the sound on all three devices simultaneously - this doesn't appear to be available for locally streamed music.

I have attempted asking Alexa to share the music to other locations but she doesn't want to play. I have also downloaded the limited PC Alexa app from the Microsoft store, but whilst it recognises the individual devices, it doesn't have the function to create a device group like the mobile app does.

Would appreciate any advice on device setup or software configuration - maybe I need to manually create a bridged group of bluetooth devices in Windows, if that's 'a thing'?

I suspect I'll need third party software to achieve this outcome.