Can I still fail a urine test if i haven't smoked marijuana for 3 days?

Yes. In fact, it's likely. If a THC is included in a drug screen, it can be detected after as long as 30 days.

THC is fat soluble. In practice, this means that remnants of THC end up in your fat cells. As your body fat level flucuates, and fat cells are consumed, traces of THC can be released into your blood stream for weeks after your last use.