You can add the Server 2008 DC to your domain while the other two DCs are running. Here are some basic steps for your migration:
1. Use ADPREP.exe to prep both the forest and the domain for server 2008
2. Promote your new server to a Domain Controllers
3. Setup DNS and DHCP
4. Once the new server is running as a domain controller, you can demote your other two servers.
Vmware on your domain controller is a strange setup, but it is possible. I recommend that you setup the Server as an ESX server running on the bare metal. Then setup your DC as a virtual server. I also highly recommend that you have at lease two domain controllers for your domain.