So I've decided to cut down my smoking to only on a weekend, and one time.
Question is, If I keep this up for 3 weeks (meaning I've smoked only 3 times), and had a drug test coming up the following week, would I be able to pass?
Theres no telling. Each persons body is different and there are too many variables to take into consideration. Only a home THC test can give you that answer.

When testing yourself, be sure to use the first urination of the day.

What can I do between the week before the test to lower THC levels?
Stop using and exercise.
However, if your home test shows a positive, and the scheduled U/A is 3 days away, then you'd want to stop exercising and be as dormant as possible until after the U/A. That way you wont be releasing a bunch of THCA into the bloodstream (as the result of fat cells being burned), which will end up in the urine to be detected.