I know this has been pretty well covered already but I still get a lot of people asking me about this one.
In windows server 2012 you can install dotnetframework 3.5 by opening the server manager and running the add features wizard, however the installation will FAIL.
To install dotnetframework 3.5 insert the windows server 2012 CD and open a powershell or CMD prompt.
now run the following command:
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess
The source is the windows server 2012 cd – in this case it is the D drive.
Hope this helps you.