I can smoke pot, and two weeks later beat a drug test. My body fat is low and metabolism is high. If your metabolism is high pot will exit your system faster, also if your body fat is low it will exit your system fast. Truly its an indivudual basis on how long pot can be detected in your system. Keep your system flushed, meaning lots of water or friut juice, and find time to sweat, toxins are realesed everytime you take a piss, and every time you sweat. So basically exercise and drink fluids in abundance.
You smoked three weeks ago, you should be just fine. The shortest time frame between me getting a drug test and smoking pot has been a week. It was in the summertime, and I past it. Even though I was nervous. I was forced to take a drug test because I had an accident on a forklift at work.