If you need .Net Framework 3.5 but Windows 10 comes with Microsoft (Dot) .Net Framework 4.5, so if you want to use .Net Framework 3.5 you need to do some things .
First you need to have a Microsoft 10 DVD version 1809 October Update or ISO image.
Mount the dvd in to the drive, iso into the virtual drive or go to the dvd/sources/sxs and copy the cab file microsoft-windows-netfx3-ondemand-package~31bf3856ad364e35~amd64~~.cab somewere on the target pc .
Click on start and in cortana search for cmd, right click on it and click run as administrator .
In the command prompt window type DISM /Online /Enable-Feature /FeatureName:NetFx3 /Source:d:\sources\sxs /All /LimitAccess and replace D: with your mounted Win10 dvd, iso or replace d:\sources\sxs with you copied path of microsoft-windows-netfx3-ondemand-package~31bf3856ad364e35~amd64~~.cab .
Type enter and you should get Enabling feature(s) [==========================100.0%==========================] The operation completed successfully.
Have fun !