Can vCenter Server 5 Essentials run vSphere 5 Enterprise Plus?

Thank you, Andrew.
So, even if I add two more ESXi Essentials hosts, the vCenter Server is limited to only 3?

Correct, you can only manage a maximum of 3 hosts using a basic Essentials license, or Essentials Plus, that's why it's so inexpensive, it's designed as a starter version of vSphere.

One more scenario, Andrew.
If I add a vCenter Server Standard, can it manage the vSphere Essentials Plus hosts?

If I add a vCenter Server Standard, can it manage the vSphere Essentials Plus hosts?

Yes, but it cannot managed Essential hosts.

If you are now moving beyond 3 hosts and essentials, it's time to upgrade to a larger versions.