Windows 8: Enable .NET Framework 3.5 (includes .NET 2.0 and 3.0) - (NetFx3) feature in Offline mode!

Inside your windows installation disk, or USB, there will be a folder named "source".

Open  it and inside it you can see a folder named "sxs"

Copy that folder to any root drive (for example, inside C:/)

Now search for cmd (Command prompt) and right click it and select run as administrator

and type command below into the cmd (Dont ignore spaces!)

Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:E:\sources\sxs

Change drive letter as you copied it.

if you have copied sxs folder to d:/ then change E:\sources\sxs  to D:\sources\sxs

and hit ENTER.

it will take some time and you can see progress in command prompt.

After it says success, close cmd
and go to control panel
left side you can see "turn windows features on / Off"
Click on it

There you can see  .NET Framework 3.5 (includes .NET 2.0 and 3.0) is checked.

Now,restart your computer! everything's perfect ;)