Is there a Messaging app that would be compatiable to:
There is some apps of third party messages app you can use in the three devices at once, go to google play and search for messages app and see which one you like and how you can use it
You can use android messaging app on all paltforms including your Windows 10 PC.
Go on your web browser on your windows PC and type in https://messages.android.com
Hope this helps!
I would suggest Whatsapp as an all around messaging app for any MMS messaging, you can download it to PC, the watch, as well as your S20+
Hope that helps!