VM Windows Server licensing in Cluster

If mix of OS editions and versions are running in VMs and hosted in multi nodes cluster. How to license Windows OS.

Scenario: if a 3 node cluster running total 12 Virtual machine with Windows Server 2008 R2 Enterprise,Windows Server 2012 Standard and Windows Server 2012 R2 Standard edition and DRS is active without any rule specification. It is virtualized with Vmware technologies.

Regards,