You can run any version of Windows without regard for the version(s) running in the other virtual machines
The versions of Windows must be no more than one release apart.
The versions of Windows must be exactly the same.
Approximately 25% fewer staffers to support the servers and operating systems.
Roughly the same number of staffers to support the servers and operating systems.
Approximately 25% more staffers to support the servers and operating systems.
Will make the introduction of a storage-area network (SAN) absolutely necessary.
Will make the introduction of a SAN desirable.
Will not materially change storage requirements.
Reduces the frequency of hardware-related service outages.
Has no material impact.
Requires that levels of redundancy be increased to avoid an increase in outages affecting multiple applications or services.
You can reboot a virtual machine without it having any effect on the other virtual machines.
If you reboot one virtual machine, all the other virtual machines reboot at the same time.
If you need to reboot one virtual machine, you have to first reboot the physical server. The individual virtual machines then reboot automatically when the physical machine reboot is finished.
It is easier to keep track of software licensing.
Tracking software licensing is neither materially easier nor harder.
It is significantly harder to keep track of software licensing.
You can (and generally should) use virtualization to put the three instances of an application or a database on the same physical server so that the development and test environments are an accurate reflection of the eventual production environment.
You should dedicate physical machines to hosting each type of instance so that the three types are not on the same server, to make it easier to secure the production environment.
It really doesn't matter where you place the different types of instances.