The difficult period of the first three days of substaining from an addictive substance or habit. It is often the case that when you're trying to avoid doing something that you are addicted to, the earliest parts of it (often three days) are the hardest. The something addictive can smoking, drugs, alcohol, fast food or even a close and intense relationship. Once the inital peroid is up, however, the quitting, abstintence and rehabilitation process is more bearable on a day-by-day basis.
"Go easy on Mike today, he's quitting cigarettes. And he's still on the second day of the three day hump."