Experience, the best teacher
If you haven't yet read it, I would highly suggest you take a look at the article "Teenage Roadtrip." I think it will provide a lot of help an insight into the kind of major undertaking you want to try.
Most importantly, I would strongly urge you to take at least one small roadtrip before you try this 3,000 mile adventure. Even taking a weekend where you and your friends just go a couple hundred miles away will give you a great deal of help figuring out each of your roadtrip styles, what things work well for your group, and where you could have some problems during your weeks on the road. Considering that you've never been more than a couple hours from home, I'd hate for you to get a thousand miles down the road only to discover you can't stand traveling with your friends and/or being cooped up in a car for 8 hours a day is something that makes you miserable.
As far as your distances, you can use 54 mph as a decent ballpark figure for the fastest you'll be able to travel over a long distance in the areas you'll be driving. That's if you are pretty much driving straight through, any stops beyond the basics for food and fuel will cause you to slow down even more.