Welcome to the Community,
ESXi hosts with a free Hypervisor license cannot be added/manager by vCenter Server.
Anyway, assuming that your 3 hosts do not have more than 2 physical CPUs each, you may want to take a look at a "VMware vSphere Essentials Kit". This Kit includes a license for vCenter Server, as well as 6 CPU licenses for ESXi (for up to 3 hosts with 2 CPus).
Since the ESXi binaries are the same for all editions, you'd only need to assign the Essentials licenses to your hosts, and then add them to the new vCenter Server.
For details like features and pricing, see e.g. Server Virtualization with VMware vSphere
André