If you are building a new environment today, chasing the 6.7 U3 ISO is often more trouble than it is worth. It is a ghost of a friendlier era in virtualization.
(Source: VMware KB 2107518 – works for 6.7 U3) Vmware Esxi 6.7 U3 Download Free Iso
If you still require version 6.7 U3 for legacy hardware compatibility, you can explore these legitimate avenues: If you are building a new environment today, chasing the 6