You should plan to be at Stansted two hours before departure, since the Stansted checkout closes 40 minutes prior to departure (no exceptions they say!).

If you use a taxi or car service or a National Express bus (I think there is some express bus service between Gatwick and Stansted) and there were no horrible traffic snarls, the drive would certainly take less than three hours. But if you are asking if you can expect to get off the plane at Gatwick and then arrive at Stansted in less than three hours, I think that would be rather risky. There is the risk of a late arrival, customs, immigration, waiting for your ride, and the actual ride to consider. (Not to mention if you might want to visit the washroom or grab a bite to eat somewhere in there.)

Check www.easyjet.com and WWW.gobycoach.com