The .NET Framework versions 2.0, 3.0, and 3.5 are built with the same version of the CLR (CLR 2.0). These versions represent successive layers of a single installation. Each version is built incrementally on top of the earlier versions. It is not possible to run versions 2.0, 3.0, and 3.5 side by side on a computer. When you install version 3.5, you get the 2.0 and 3.0 layers automatically, and apps that were built for versions 2.0, 3.0, and 3.5 can all run on version 3.5. However, the .NET Framework 4 ends this layering approach.
NET Framework 4.5 is included with Windows 8, the .NET Framework 4.5.1 is included with Windows 8.1 and the .NET Framework 4.6 is included with Windows 10.
Installing .Net 3.5 on Windows 8 and 10 is possible via "Control Panel", click on "Turn Windows features on or off" under "Programs and Features".