When you install .NET 3.5, you are actually installing a "package deal" that includes:

Installing is the official way to support legacy applications that require versions 2.0 or 3.0 . Microsoft designed the 3.5 release as a "cumulative" update, meaning it includes all the core components of its predecessors. 🧩 Does .NET 3.5 include 2.0 and 3.0?

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

Are you running into a while trying to install it? Microsoft .NET Framework 3.5 Service pack 1 (Full Package)