My customer had an issue installing .Net 3.5.1 on Windows 10 today. The system is on an airgapped network and couldn't get to Windows Update to retrieve the source files.
The solution was to install the feature using Dism, and specify the DVD as the source media. The command is:
Dism.exe /Online /Enable-Feature /FeatureName:NetFx3 /Source:D:\Sources\SxS