You can find this file inside the original Windows installation media (ISO or USB). NET Framework 3.5 installation errors - Windows Client
Once you have located the file—either by extracting it from an ISO or downloading a standalone version from a trusted repository—you can install it using the DISM command. This is a powerful administrative tool, so proceed with care. Microsoft-windows-netfx3-ondemand-package.cab -Extra
For enterprise air-gapped systems, download the official CAB from the or Visual Studio Subscriptions . Never trust public file-sharing sites. You can find this file inside the original