As with any release, it is important to read the release notes...there are often some hidden nuggets of information that you need to know.
That is certainly the case with vSphere 6.0 Update 3 or Update 3a, especially if you are thinking about upgrading to vSphere 6.5 after that in a short timeframe.
According to the release notes for 6.0 Update 3, http://pubs.vmware.com/Release_Notes/en/vsphere/60/vsphere-vcenter-server-60u3-release-notes.html, and 6.0U3a, http://pubs.vmware.com/Release_Notes/en/vsphere/60/vsphere-vcenter-server-60u3a-release-notes.html, it is currently not supported to then upgrade to 6.5 from either release.
Here's the important snippet from the release notes:
Upgrade Notes for This Release
Upgrading from vCenter Server 6.0 Update 3 to vCenter Server 6.5 is not supported.
Update: It is now possible to upgrade from 6.0U3 to 6.5U1 which was released on July 27,2017.