Troubleshooting .NET Framework 3.5: Your Guide to the NetFx3-OnDemand-Package.cab
Microsoft does not provide a direct standalone web download for this specific Microsoft-windows-netfx3-ondemand-package.cab Download
If you are in an "air-gapped" environment (no internet) or dealing with a PC that has Windows Update restricted by group policy, you need the .cab file. Where to Download the .NET 3.5 CAB File Troubleshooting