How much damage is caused by smoking 2-3 cigarettes per day on a healthy body?
Drugs have the property of "tolerance." Nicotine is no exception. It is "Progressive diminution of the susceptibility . . . to the effects of a drug, resulting from its continued administration." That means the expected effects of the drug seem to become weaker with use and more is needed to achieve the accustomed effect.
Nicotine's tolerance is slow compared to other drugs such as heroin with tolerance occurring over the course of a few days or weeks. Nicotine takes months or years.
To answer your question, the greatest harm from a few cigarettes a day is that you will find yourself needing more in the near future in order for the experience of those few now. With nicotine it's very gradual, almost unnoticeable but relentless. You may try to limit yourself to those few, but you will find that extremely difficult if not impossible.
There are plenty of reasons not to smoke. This is just one of many. No one plans to become a heavy smoker but it happens. The habitual heavy smoker not only has the worry of serious harm to one's health but also the enormous expense, the difficulty because of smoking bans of finding a place to smoke, and isolation resulting from social disapproval.