The environment is Zenworks 11.3.2, installed on 3 primary servers
running Windows 2008 R2 Enterprise (64bit).
External SQL database

The plan is to migrate three zenworks servers to Windows Server 2012 R2 Standard

The way to do this is via the disaster recovery procedure.
"Replacing an existing Primary Server with a
New Primary Server"

What I have to do?
Stop Services on all three Servers
Replace the first server
Second server and so on
Stop Services on the first primary
Replace the first server
Stop Services on the second...