Hi there and thank you for your question. I can appreciate your concern regarding this issue and I ll try help where I can. Judging by your age and weight as well as your frequency of smoking, the chances of you passing a drug test after stopping smoking marijuana for 3 weeks is around 50%. This would include the factors that you were previously smoking regular grade marijuana and this was done 3-4 times a week over the last 20 years. In this case, for you to get as close as possible to a 100% pass on your drug test, you would need to be marijuana free for around 6 weeks. With all this being said, if you can leave the drug alone for 6 weeks then you might as well stop using it then. I not going to advocate on your life style choices, but I would like to offer the advice from a medical perspective. I do feel that there is a place for THC medications for the correct indications but in healthy individuals, there can be frontal lobe issues which causes personality changes and one can come into trouble with the law when found in possession of this drug. I hope I have been able to answer your initial question though.