Box 1: No -
Group1 already has a cloud endpoint named Share1.
A sync group must contain one cloud endpoint, which represents an Azure file share and one or more server endpoints.
Box 2: Yes -
Yes, one or more server endpoints can be added to the sync group.
Box 3: Yes -
Yes, one or more server endpoints can be added to the sync group.
Reference:
https://docs.microsoft.com/en-us/azure/storage/files/storage-sync-files-deployment-guide