Starting with Windows 8, .NET 3.5 comes more or less tightly coupled to the OS, as a Windows Feature. But there's a shortcoming: Even though one doesn't need to download an installation package, the Windows Feature dialog is supposed to download it via Windows Update. This can lead to serious troubles, if your computer is joined to a domain and the sysadmins did not add the .NET Framework 3.5 to the WSUS storage.
Why this Reference
I have found myself in a situation where Windows was unable to install .NET 3.5 even on newly setup machines over the UI. It was always a major PITA to go and grab the command line I needed from the depths of the internet.
The Command Line
Luckily enough, the .NET 3.5 CAB package is present on the Windows 10 installation media. To install the CAB file, open up an elevated command prompt and use the following command:
dism /online /enable-feature /featurename:NetFx3 /All Source:<<DRIVE>>\sources\sxs /LimitAccess
<<DRIVE>> with the drive letter of your Windows 10 Installation image, e.g., if your image is inserted into drive D:, use:
dism /online /enable-feature /featurename:NetFx3 /All Source:D:\sources\sxs /LimitAccess