Dude, it's been three weeks? You must have the slowest metabolism on the planet. If you're still testing positive, then it would probably be easier to pass with dilution. Here's what you do:

Go get a big jug of cranberry juice. Drink it all the night before and then after it's empty, fill the bottle and drink again. Repeat this as much as possible while taking pisses. The day of the test, make sure to take a pack of multi vitamins (I like the GNC megamans). You should be fine.

BE CAREFUL THOUGH. It is possible to drink too much water and die.