Installing can be a headache, especially when Windows Update fails or you’re working on an offline machine. The most reliable "pro" method is using the CAB file (the cabinet file containing the framework payload) and the Deployment Image Servicing and Management ( DISM ) tool. Where to Find the CAB File
If you don't have the media, Microsoft's Deployment Considerations suggest using the folder from a matching Windows version ISO. How to Install Using the CAB File cab file for .net framework 3.5
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:D:\sources\sxs Alternative: Online Installation cab file for .net framework 3.5