When you enable the .net 3.5 framework on Windows 8 or Windows 2012, Windows downloads the installation files before starting the installation. However, when you do not have an internet connection this procedure does not work, and you must specified the location of the Framework installation files. These files are available on the installation DVD of Windows 8 and 2012. Simply insert it and use the following command:
dism.exe /online /enable-feature /featurename:NetFX3 /Source:D:\sources\sxs /LimitAccess
D: with the letter of the drive containing Windows files.