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.
Whats should I do as my 6.7 VC's are all at 6.7 3g - wait I guess ?
Yes, waiting for a supported version is the only option. There may be workarounds, but you probably won't use them in a production environment, which need to remain in a supported stata.