Will second hand weed show on a hair test three weeks ago?

No, it won't, because only a trace will have settled in the hair, and will have been washed out. The hair test detects THC which has been deposited in the hair from the body, so it can be proven that a smoker has stopped taking cannibis, and how long ago that was.The test is not sensitive enough to detect the tiny amount you would have inhaled while you were in the car.