That's just conjecture on your part. Some people are lucky, others are not, it's not always due to behavior. 3 cigarettes a day is also extremely low - the average is apparently 14[1] right now.
[1] https://www.cdc.gov/media/releases/2018/p0118-smoking-rates-...
I don't believe there is any harmful information but the whole picture is that most folks cannot smoke a few per day. Eventually cigarettes will own most smokers.
Nicotine, CO and radioactive toxicity have a long half-life, where primary radical damage or reactive carcinogen burden is dose dependent stochastics.
Also people forget, you are much, much more likely to die from cardio-vascular damage or COPD as a consequence of smoking, than lung cancer.
The addictive nature of cigarettes makes it very hard to stick to 3 a day.