In the Windows Server hybrid administrator objectives for AD DS, application directory partitions (also called "naming contexts") are forest-wide objects whose creation and deletion are controlled by the Domain Naming Master FSMO. The AZ-800 materials explain that operations which "add or remove naming contexts in the forest" must be performed on, or reachable by, the DC that holds the Domain Naming Master
; if that role holder is unavailable, the operation fails until connectivity is restored or the role is transferred
/seized. Because your custom application partition belongs to the forest schema of naming contexts, removing it requires contacting the Domain Naming Master. From the table, DC3 holds the Domain naming master role. Therefore, a failed attempt to delete the partition indicates the Domain Naming Master is not available, which points to DC3 being unavailable.