If the standard "Turn Windows features on or off" menu fails, you can manually install the .cab file using the tool.
DriveLetter:\sources\sxs\
So you search for a solution, and you stumble upon a file named microsoft-windows-netfx3-ondemand-package.cab . But then you see a variation: microsoft-windows-netfx3-ondemand-package.cab -Extra . Microsoft-windows-netfx3-ondemand-package.cab -Extra
Sometimes, DISM refuses to work due to a corrupted servicing stack. In these deep-corruption cases, you must go "Extra" – manually extract the CAB and place files. If the standard "Turn Windows features on or
DISM /Online /Add-Capability /CapabilityName:NetFx3~~~~ /Source:X:\sources\sxs /LimitAccess Microsoft-windows-netfx3-ondemand-package.cab -Extra
What is that -Extra suffix? Is it a hack? A dangerous mod? Let’s clear the air.