Tips & Tricks : How to add .NET Framework 3.51 to Windows 8 or Windows Server 2012
Unfortunately there is a problem when you try to add .Net Framework 3.51 to Windows Server 2012 using Server Manager. In order to fix that issue and get .Net Framework 3.51 installed (which is required by majority of applications and servers) issue following command from Command Prompt:
dism /online /enable-feature /all /featurename:NetFX3 /source:x:\sources\sxs
Please notice that x: in source parameter has to be replaced with appropriate drive letter for DVD with Windows Server 2012 installation files.