It would seem extremely simple to install .net framework 3.5 on Windows 2012 since it is among the features to be installed (exactly as in Windows 2008 R2); in fact if you try to install it the installation will fail cause it cannot find the sources to be installed because, by default, they are not copied on hd during installation of the OS but they still exist on the Windows installation dvd (a warning to insert the Windows dvd would have been too simple….)
To install it go to “Add Roles and Features” and select the component .net 3.5:
On the next screen click on the option “Specify an alternate source path”:
and as source path specify the path on the windows DVD “D:sourcessxs”