As stated in the 7.0.0a release notes - upgrading from 6.7 3g is not supported.
Whats should I do as my 6.7 VC's are all at 6.7 3g - wait I guess ? or has anyone tried an upgrade to 7.0.0a and it works ? It just isnt a supported upgrade ?
Extract from release notes below :-
IMPORTANT: Upgrade from vCenter Server 6.7 Update 3g to vCenter Server 7.0.0a is not supported. For more information on vCenter Server versions that support upgrade to vCenter Server 7.0.0a, see VMware knowledge base article 67077.