For Windows 7, Vista, or XP, you can download the "Full Package" on a connected computer and transfer it via USB. Install .NET Framework 3.5 on Windows 10 - Microsoft Learn
If you have a Windows 10/11 ISO or USB installation media, the .NET 3.5 SP1 sources are included in the sources\sxs folder as microsoft-windows-netfx3-ondemand-package.cab . This is the most reliable offline source for modern Windows. net framework 3.5 sp1 offline installer