Way to go joinig up. Id never do it, but i gotta love those who do
Im sorry about your friend
As for cleaning out your system
Ive got a whole bunch of experience with drug tests and how to fool em'. With three weeks you should be fine, espeacially with waldo's advice of running and water. But, if you happen to fall off the wagon again and need a real quick way to flush out your system there is one fool proof desprate measure. I warn you its not pleasent, but it works well.
You need to take about 20 asprin and drink 2 gallons of water. This works very well, and if you drink warm water and dont run around too much for about 8 hours afterwards, youll be fine, just a little tummy ache. If you make my mistake and drink cold water and then go for a 2 mile jog in central park, you might go into mild shock.
But try not to have to do that, its not particularly healthy.
Have fun in the military, and thanks for doing what must be done.