How to install .NET Framework 3.5 on Windows Server 2012 and Windows Server 2012 R2
.NET Framework 3.5 needs the source files off the Windows Server 2012 DVD in order to install. You can do this online and without needing a restart using DISM.
1. Go to a command prompt and enter this:
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:sourcessxs /LimitAccess
Note: Source should be the Windows DVD location, ie d: /Source:x if x is your DVD mount point.