There's a .NET Framework 2.0, but no 3.0 that I'm aware of. Assuming you just put down the wrong version number... There is no real point to the .NET Framework for the majority of people. There are a few apps that require it to run, but for the most part, it's still aimed more at developers than users. For users, it's about 150MB of disk space wasted, and some possible alarm over the sudden appearance of the ASP.NET user account... Well, concern for the users that actually notice that, which is only a small fraction I'm afraid.

Anyway, unless you've got some program you want to run that requires the .NET Framework, don't bother with it.