One of d most simplest method of enabling this feature is true the Windows installer flash/disk or Windows iso file
Windows 8 does not include .NET 3.5 (include .NET 2.0 and 3.0) by default. But if the user is upgrading from Windows 7 to Windows 8, .NET Framework 3.5 is fully enabled. The user can make sure this feature is enabled by check in Program and Features.

These is list of steps that the user can do to check it :

You still can enable this feature in offline mode if you has Windows 8/8.1 DVD or ISO image or Windows 8/8.1 usb installer. Read more on how to create iso image  and windows 8/8.1 usb installer

The following are the steps to enable .NET Framework 3.5 (include .NET 2.0 and 3.0) feature in offline mode :

Step 1 :
Insert Windows 8/8.1 DVD or mount ISO image or the usb installer. The source of this feature can be found in folder E:\sources\sxs. (In this case E: the user’s drive letter on which the user has loaded Windows 8 Media.)

Step 2 :
Open CMD.EXE with Administrative Privileges.
Or press windows key+ x and select command prompt with administration.
Step 3 :
Run the following command <b>Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:E:\sources\sxs /LimitAccess </b>, and hit Enter.

After completing the installation of .NET Framework 3.5 you can see that the feature is enabled. By pressing windows key + x And select program and features