Hello @Snookred ,
Thank you for posting here.
Based on the description above, I assume you have four logic drivers on the machine.
A driver: OS1
B driver: OS2
C driver: OS3
D driver: Public data that OS1 and OS2 and OS3 can all access the public data on drive D.
Public data: what I'd call user work product data (i.e., documents, pictures, videos, music, etc.) is stored on a separate drive that is shared amongst all (typically 3) instances of Windows that can be independently booted.
Then you need to back up A and B and C respectively.
And you only need to back up D drive one time.
Hope the information above is helpful.
Should you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou