The specific file required is typically named microsoft-windows-netfx3-ondemand-package.cab . You can find it by:
Understanding when to use the CAB file instead of the standard .exe installer is crucial: cab file for .net framework 3.5
Why .NET 3.5 is needed (legacy apps, SQL Server, etc.) and the limitation of online installation in air-gapped environments. cab file for .net framework 3.5