I only smoked weed once and it's been 3 weeks can I pass a urine test?

Please check this page, myth 4: http://www.erowid.org/psychoactives/law/law_article5.shtml

Basically as long as you don't smoke again and you're only a first time user, depending on your metabolism and other factors it could be out of your system in as little as 3 days. I would say it would take about 1 to 2 weeks maximum to get out of your system. If you're a chornic user it takes about a month to get out of your system.