If you try to install .NET Framework 3.5 on Windows Server 2012, using the Server Manager and accepting the defaults, you will get an error stating that the source files could not be found. During the installation process Windows Server 2012 does not copy the required files for installation of .NET Framework 3.5 as it did in Windows Server 2008 R2. On the confirmation screen after selecting the .NET Framework 3.5 Features, you’ll notice a yellow bar telling you that an alternate path to the data files needs to be specified. Near the bottom of the window is an option to Specify an Alternate Source Path. Click this and specify the following path, substituting X with your optical drive with the Windows Server 2012 media:
Now the installation should complete without any issues.