Update:
Tried restarting the VM, Shutting down the VM, removing from inventory and re-adding. None of these helped.
Patched vCenter from 5.1 to 5.1 U1a and rebooted the VC. Attempted to vMotion one of the troublesome VMs - same issue.
About 2 hours later the system successfully vMotioned one of the VMs due to a DRS recommendation. All 3 VMs now vMotion successfully.
I'm not sure what the cause was but everything is back to normal now.
Kyle.
