Depending on the patch status of the WSUS servers, you may not be able to install .NET 3.5 on Windows 8 or Windows Server 2012.  To fix this via GUI, follow these steps:​

Mount the .iso or insert the dvd for Windows 8 or Server 2012, whichever one matches the OS on your computer.​

Server 2012

  1.  Go to server manager and run the Add Roles and Features wizard
  2.  Hit next, accepting all defaults, until you reach the ‘Features’ page
  3.  Select .NET Framework 3.5 Features, then select Next
  4.  Select Specify an alternate source path
  5.  Enter the path to the SxS directory
         Ex:  D:\Sources\SxS\
         Ex: \\ServerName\ShareName\Sources\SxS\
  6.  Select OK, Install

Windows 8 or Server 2012

  1. Insert the Windows 8 or Windows Server 2012 installation media.
  2. At an elevated command prompt, run the following command:
    Dism /online /enable-feature /featurename:NetFx3 /All /Source:<drive>:\sources\sxs /LimitAccess

    Note In this command, <drive> is a placeholder for the drive letter for the DVD drive or for the Windows installation media. For example, you run the following command:

    Dism /online /enable-feature /featurename:NetFx3 /All /Source:D:\sources\sxs /LimitAccess