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This confused me too for a while. I believe that since update 3h only includes Security Fixes for Photon OS the release notes are on a separate page.
Check this link and scroll to the bottom and you'll find a reference to "6.7 U3h" - https://docs.vmware.com/en/VMware-vSphere/6.7/rn/vcenter-server-appliance-photonos-security-patches.html#update3
in addition to the answer from Scissor you may also find it useful to read this, which concerns the 6.7U3h:
We are faced by this problem with a couple of vCenter installation. First time we thought that we have a wrong password and reset it but than take notice about the stopped service.