Please follow the official steps from a Technet blog post to enable online and offline activation for .NET 3.5. You will need to have an installation DVD nearby because you will need it.
For a fast way of activating it offline, please follow these steps.
- Insert the installation DVD
- Open a command prompt with elevated access and type : dism /online /get-features |more to check the state of the netfx3 package
- Use this command ** xcopy d:\sxs*.* c:\sxs /s** (D is the letter of your DVD-ROM, replace it with the one of your DRIVE if necessary)
- Wait for the copy process to finish, then type dism /online /enable-feature /featurename:NetFx3 /All /SourceC:\sxs /LimitAccess and your .NET 3.5 will be activated offline.