If I have .NET Framework 2.0 and .NET Framework 3.5, do I need .NET Framework 3?-Collection of common programming errors

.Net 3.0 introduced WCF, WPF and WF, extra libraries really working on top of the 2.0 runtime.

.Net 3.5 introduced upgrades to these libraries and comes with support for Linq (new language features).

You can safely uninstall 3.0, and replace it with 3.5, as 3.5 covers the libraries introduced in 3.0. Check out this page at wikipedia for more information on the versions and what they bring to the table.

When you install 3.5 you will get a 3.0 folder in your framework folder (along with all other versions).