Box 1: No -
A sync group must contain one cloud endpoint, which represents an Azure file share and one or more server endpoints.
Box 2: Yes -
Data2 is located on Server2 which is registered to Sync1.
Box 3: No -
Data3 is located on Server3 which is not registered to Sync1.
Reference:
https://docs.microsoft.com/en-us/azure/storage/files/storage-sync-files-deployment-guide?tabs=azure-portal%2Cproactive-portal#create-a-sync-group-and-a- cloud-endpoint