While enabling .NET 3.5 enables legacy software, it introduces maintenance considerations. .NET 3.5 relies on .NET 2.0 and 3.0 subsystems. Security updates for .NET 3.5 are delivered via Windows Update.
The Deployment Image Servicing and Management (DISM) tool is the standard utility for servicing Windows images. This is the recommended approach for offline installation. net framework offline installer 3.5