Sorry, I still don't exactly understand your current situation. What I understand until now is that a vCenter Server with 3 connected hosts has failed and you are trying to add these three hosts to another vCenter Server (Essentials Plus) which already has 3 hosts connected!? This won't work, because vCenter Server for Essentials is limited to 3 hosts. If you have another license - which was assigned to the failed host - you may use this to setup another vCenter Server to which you the can add the hosts, provided you have enough CPU licenses for them.
André