I don't know much about methadone either, but can you possibly run it by your doctor? Don't forget, this is not advice, just something I've read and has been an experience, you can withdraw too quickly too and that can cause trouble too. I know I'd feel better if you could call and advice nurse or doctor. We just want to make sure you stay around!!!
Three weeks might be reasonable, but it may be too quickly too. Take good care of yourself and if you start to feel funny or anything, please promise to get to an ER.
Good luck.