vCenter 6.0 Update 3a Released
VMware has released vCenter 6.0 Update 3a, this fixes a critical security issue, see here: http://www.vmware.com/au/security/advisories/VMSA-2017-0004.html
See full release notes here: http://pubs.vmware.com/Release_Notes/en/vsphere/60/vsphere-vcenter-server-60u3a-release-notes.html
Upgrading from vCenter Server 6.0 Update 3 and 6.0 Update 3a to vCenter Server 6.5 is not supported.
We are at Update 2
Is it required to move to Update 3 prior to moving to Update 3a.
No, you can just upgrade from update 2 to update 3a.
Do you know, why you cannot upgrade to vSphere 6.5?
No, but probebly to do with changes in the databases or API, that need to implemented i vSphere 6.5 for the migration/upgrade.