We have a virtual environment with 3 hosts that I recently purchased Data Center licenses for.  I know that with these licenses I can spin up as many VMs as I would like, and that would allow me to configure fail-over and not have to worry about being out of compliance with licensing.  What I dont know is how I assign those licenses to my ESXi hosts so that each one gets a license, and I dont end up with all 3 on the same host. 

Thanks in advanced for your help. 

In terms of assigning a license for Microsoft products, assignment means making and keeping records. You can do that any way you like, even writing it down on paper is fine.

e.g.:
host-hv1  -  license ABC1
host-hv2  -  license DEF2
host-hv3  -  license GHI3

Assigning a license is a requirement, how you actually execute the requirement is up to you.

If you are using VMware ESXi as your hypervisor, and you have three hosts, and those are clustered so that workloads can move across hosts dynamically as required - none of that has anything to do with license assignment of your Windows Server Datacenter Edition (DCE) entitlements - because you only need to have DCE assigned to a host and that allows you to run the unlimited guest workloads on that host, you don't need to have more than one DCE assigned. (unless you have more than 2 CPUs in a host).

Assignment isn't an electronic thing, so, you can't "end up with all 3 on the same host", unless you write it down on paper like that.

Or have I misunderstood your question?

Thank you for your reply. 

You are understanding me correctly.  the part I'm having trouble with is how I actually go about assigning the DCE to a host. Do I simply run a VM on each with that license and from there am allowed to move them around, or is there more to it than that?

I'm sure I'm overthinking this, but I would like to understand it before I start handing out these licenses.

the part I'm having trouble with is how I actually go about assigning the DCE to a host. Do I simply run a VM on each with that license and from there am allowed to move them around, or is there more to it than that?

Assignment is a record that you keep - "I installed [this] on [that] host".
Technical steps are as simple as creating the VM in ESXi, installing Windows Server within that VM, inserting the DCE product key to that installation.

Repeat for each host and each VM.

If workloads move across/between hosts, your assignment is still valid (unlimited VM workloads is the DCE benefit).

e.g.: write down your assignment "I installed DCE on host-hv1 & installed DCE on host-hv2". create 8 VMs on host-hv1. Install DCE in each VM. Move all 8 VM workloads from host-hv1 over to host-hv2. A this point you have no further records to keep, nor any licensing tasks to perform, because host-hv1 still has DCE assigned to it but you aren't using it just now, and, host-hv2 still has DCE assigned to it and it has 8 VMs running on it, which is permitted by the DCE entitlement. You can continue to move VM workloads between hosts without any further worry, as long as the host is assigned a DCE license.

Perhaps you are overthinking this ;)