How long should I expect to be feeling nicotine withdrawal symptoms if I quit three weeks ago?

Usually about 3 wks. Nicotine withdrawal symptoms can be diminished by using nicotine replacement drugs (gum, patch, inhaler, nasal spray) and/or other pills (bupropion, varenicline). Talk to your doctor, get a date on the calendar, alert your friends & family that you need their support, and do yourself the biggest favor of your life and quit (if you have not already). Good luck.