You should go down to half dose every other day for a week, then every third day for a week, then stop. Tell your doctor what you are doing.
I am not a doctor but do have experience with getting off Xanax. First off listen to the Doc. tell your Doctor what you are doing. Don't beat yourself up if you can't go down the full half dose. I sent down a quarter dose for one week then another quarter dose the second and third week then another quarter dose the forth and fifth week. Then took the last quarter every day for one week then started skipping days. If is NOT EASY but you have not been on it too long so it should not be that bad. I kept a list of normal withdraw symptoms to know what I was experiencing and could refer to it when I was panicking to reinforce that what I was going through was normal. I was on it alot longer than 3 weeks and it was the most difficult thing I ever did. Don't want to scare you but the first 3 days without it I spent in bed all day screaming into my pillow in pain. YOU CAN DO IT. I have been without Xanax for 5 years! Easier now than later.