As of vSphere 6.7 VMware no longer supports VMFS version 3 datastores.
If a vSphere ESXi 6.7 host mounts a VMFS3 datastore, it will be automatically upgraded to VMFS 5.
This automatic upgrade process is in-line and does not impact the availability of virtual machines, however this upgrade process will not expose all VMFS5 features to ESXi / virtual machines (The block size change is one feature not included in the upgrade)
To take advantage of all VMFS features, you need to migrate all VMs on the VMFS datastore to a new datastore, delete the old datastore and create it again, specifying the correct VMFS version
One other note, is that when installing ESXi and specifying a boot LUN, you will see this notice regarding VMFS3 upgrades: