microsoft-windows-netfx3-ondemand-package.cab file is the offline installer package required to enable .NET Framework 3.5
Microsoft provides the ADK for free. Within the installation media, you can extract the necessary FOD (Features on Demand) CAB files. microsoftwindowsnetfx3ondemandpackagecab download free link
You can copy it from there to your desktop. But again – it’s tied to that specific Windows build. microsoft-windows-netfx3-ondemand-package