While Microsoft hosts the original 3.5 SP1 full package, it is designed primarily for Windows XP, Vista, and Server 2008.
Install-WindowsFeature Net-Framework-Core -Source D:\sources\sxs .net 3.5 sp1 offline installer
If you are trying to run legacy enterprise software or older applications on a modern Windows machine, you have likely encountered the dreaded "requires .NET Framework 3.5" error. While Windows 10 and 11 have moved on to newer versions, many essential business apps still rely on the 3.5 framework (which includes SP1). While Microsoft hosts the original 3