I hate to be the one to tell you, but the only thing that really works is to drink a BOATLOAD of water (8+ glasses per day....I know it's more than you feel like you can take, but it's the only way), excercise with wild abandon (we're talking at LEAST an hour a day of pushing yourself to your limit), and avoid things that slow your metabolism, like heavy foods and "downers."
"Clean out" potions are BULLSHIT and don't do crap. The only thing that works is to move lots of fluid through your system, aggressively stimulate your metabolism with excercise and good nutrition, and cross your fingers.
Or stick it to the man and don't submit to tests. But, of course, not all of us have that option.
Good luck! Don't stress, stick to the plan and you'll be just fine. Three weeks should be just enough time if you don't laze out and try to save yourself at the last minute with placebo bullshit ripoff formulas or try to fake your way through the test with purchased urine or some such. That usually doesn't work either.