You don t need to migrate your VMs off the ESX servers at all. You can change the ESX IP address on the fly - just remove it from the vCenter, change IP and add it back again. I would do the following:
1. Disable HA;
2. Remove all 3 ESX from vCenter;
3. Change IP of all ESX servers;
4. Change IP of vCenter;
5. Add all ESX backwith new IPs;
6. Turn on HA.
This should work.
Cheers;
Marcelo Soares
twitter: @mtsoares42