Run sfc /scannow in Command Prompt to fix underlying OS issues before attempting installation.

Download Microsoft .NET Framework 3.5 Service pack 1 (Full Package) from Official Microsoft Download Center. Download Center.

This report provides a comprehensive overview of .NET Framework 3.5

If you have the Windows installation ISO or USB drive mounted as drive D: , use the DISM tool:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:D:\sources\sxs

Basic runtime for older 2000s applications.

and compiler improvements (C# 3.0), while continuing to use the same core 2.0 runtime engine. Stack Overflow Install .NET Framework 3.5 on Windows 10 - Microsoft Learn