Can't install .net 3.5 on Windows 10

Please open a command prompt (cmd.exe) as admin and run the following command to install .Net 3.5:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /Source:d:\sources\sxs /All /LimitAccess 

(replace D: with your mounted Win10 ISO)

In some environment, DISM fails to fetch the file (Proxy, network policies). The DISM command takes the files from the DVD and you're fine.

If you are MSDN subscriber you can also download the Windows 10 Features on Demand ISO, mount the ISO, copy the Microsoft-Windows-NetFx3-OnDemand-Package.cab to C:\Features (or a different folder) and those commands:

DISM.EXE /Online /Add-Capability /CapabilityName:NetFx3~~~~ /Source:C:\Features

If this fails, run this:

DISM.EXE /Online /Add-Package /PackagePath:C:\Features\Microsoft-Windows-NetFx3-OnDemand-Package.cab

Windows Side by Side (WinSxS) location is probably empty in the registry.

From the GUI called from APPWIZ.CPL this results in error code: 0x800F081F

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Servicing]
"LocalSourcePath"="d:\\sources\\sxs"

Check/add the subkey 'Servicing', and the value of 'LocalSourcePath'. It should contain a correct string (e.g. 'd:\sources\sxs'), data type (REG_SZ). Where "d:" represents your Windows media drive letter.

I was stumped when this was failing as well but then found out I had a group policy blocking me from enabling windows features. gpedit.msc >> Computer Configuration >> Administrative Templates >> System >> Specify settings for optional component installation and component repair. Set to Enabled and check Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS) - source:https://www.askvg.com/fix-cant-install-microsoft-net-framework-3-5-in-windows-8-and-later/

From strikeforcezero comment

This may be another option:

https://sourceforge.net/projects/framework-3-offline/

Uploaded on 2016-06-27 - last comments were in 2019. Appears to work with an air-gapped computer.