Offline installation of .NET framework 3.5 on windows 10

1. Insert your Windows 10 DVD, or double click its ISO image, or insert your bootable flash drive with Windows 10 depending on what you have.
2. Open ‘This PC’ in File Explorer and note the drive letter of the installation media you have inserted. In my case it is disk D.
3. Now open an elevated command prompt and type the following command.

Dism /online /enable‐feature /featurename:NetFX3 /All /Source:D:\sources\sxs /LimitAccess

Replace D: with your drive letter for Windows 10 installation media.