Thanks. But looks like the pages only classified those features different vmware product have or not, and not relating exactly what I am asking.
from the licensing perspective it makes no difference, which 6.x version you are installing on the third host, 6.0, 6.5 or even 6.7, and all update versions.
As long as you have not more than 3 hosts which max 2 sockets each.
If you have a vCenter installation for management, check compatibility first.
Also consider vmware.com/go/hcl for hardware compatibility.
The only thing you might have in mind is the country restrictions of the license agreement (see EULA) if the third host is in another country.