Hi,
Based on my understanding, there are 2 DCs (DC1 and DC3) in your domain, right?
Do you mean you want to demoted DC1 and remove it from Domain, but the policies sitll show applied from DC 1?
Or both the DCs (DC1 ,DC3) are working well, you just want the policies apply from DC3
If you mean you demoted DC1 and remove it from Domain, but the policies still show applied from DC 1? You can confirm the following steps
If there are FSMO roles in the DC1, you may try to transfer the FSMO role first:
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/view-transfer-fsmo-roles
For how to demote one DC, you can refer to:
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/demoting-domain-controllers-and-domains--level-200-
If the DC1 was not demoted successfully, we need to perform a metadata cleanup.
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/ad-ds-metadata-cleanup
Then, the client will apply policies from DC3 since DC1 is not existing anymore.
If both the DCs (DC1, DC3) are working well, you just want the policies apply from DC3, you can try to confirm the site configuration.
Did the 2 DCs are in the same site or different sites?
If in different sites, the user or computers will find the DC in the same site.
If in the same site, the user or computers will apply policies randomly.
The process is allowing the computers on the network to find the closest domain controller to retrieve the Group Policy information from SYSVOL. This behavior is to load balance and synchronize fault tolerant between domain controllers.
Following link for your reference:
https://social.technet.microsoft.com/Forums/windows/en-US/a23d914c-6998-4107-af3d-db2d380693d1/change-the-server-group-policy-was-applied-from?forum=winserverGP
If i misunderstand you, please feel to let me know.
Best Regards,