How Long Does Marijuana Stay In Our Body?

I have to take a drug test for a new job in 3 weeks I quit smoking weed when I got the job 3 days ago but I have smoke weed almost everyday for about 5 years...I am 5 8 and I weigh 158lbs. I m a little worried it won t be completely out of my system by then. Will taking niacin help with this? If so does it matter if it s flush free or not?