Understanding .NET Framework 3.5: The Inclusion of .NET 2.0 and 3.0
If you're using Windows 10 or later, .NET Framework 3.5 is not enabled by default. You can enable it through the Turn Windows features on or off dialog:
Windows 10 and later:
In the list, look for .NET Framework 3.5 (includes .NET 2.0 and 3.0). Check the box and click OK.
Key Features of .NET Framework 3.5
Understanding .NET Framework 3.5: Why It Includes 2.0 and 3.0 (and Where to Download)
You can download .NET Framework 3.5 from the official Microsoft website: net framework 35 includes net 20 and 30 download link
What is .NET Framework 3.5?