Wondering if to enable vCenter HA require 3 node, active, passive and witness. what if 2 server compatibility server and plus 1 old server as witness can HA be done?
The requirement is 3 datacenter objects "managed by vCenter". Is your third option managed by vCenter? If not, then not likely this is going to work.
https://kb.vmware.com/s/article/2148003
There is an option used for labs (not supported or recommended for production) to get around this requirement. So, if you are asking for the purpose of a lab, this may be an option.
How to enable vCenter Server High Availability (VCHA) in vSphere 6.5 w/single ESXi host? (williamlam.com)
Yes. It is possible.Instead of the automatic method, you can proceed with the manual method.