Open Notepad then copy in this code
@echo off
Title .NET Framework 3.5 Offline Installer
for %%I in (D E F G H I J K L M N O P Q R S T U V W X Y Z) do if exist "%%I:\sources\install.wim" set setupdrv=%%I
if defined setupdrv (
echo Found drive %setupdrv%
echo Installing .NET Framework 3.5...
Dism /online /enable-feature /featurename:NetFX3 /All /Source:%setupdrv%:\sources\sxs /LimitAccess
echo.
echo .NET Framework 3.5 should be installed
echo.
) else (
echo No installation media found!
echo Insert DVD or USB flash drive and run this file once again.
echo.
)
pause
Save the file as netfx.cmd
Right click it and choose Run as administrator.
The file will install .NET Framework 3.5 automatically after finding the drive letter of your Windows 10 installation media.
If media not found insert Win10 Usb or Dvd drive and run as administrator again.