Installing .NET Framework 3.5 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
sources\sxs folder.microsoft-windows-netfx3-ondemand-package.cab.C:\Temp\) on the target machine or a network share.The .cab file serves several purposes:
Do not download CAB files from third-party websites. These files can be tampered with, contain malware, or be for different language/architecture versions, leading to installation failures or system corruption. Always use Microsoft-signed sources. cab file for .net framework 3.5