Hello,
Thank you for posting in Q&A forum.
To resolve this high workload issue, you can try to configure other 2 DNS servers as primary DNS server. If you are worried about that these 2 servers are not able to resolve part of domain names, can add another one as DNS forwarder for these two.
For Microsoft Official Load Balancing resolution, please kindly refer to below link:
https://learn.microsoft.com/en-us/windows-server/networking/dns/deploy/app-lb
Hope this answer can help you well.
Best regards,
Jill Zhou