Offline install of .NET Framework 3.5 in Windows 10 using DISM
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 command prompt as administrator and type the following command:
Dism /online /enable-feature /featurename:NetFX3 /All /Source:D:sourcessxs /LimitAccess
(Note: D:sourcessxs is location of where you extract ISO image)