In the same way that some individuals can die from a bee sting or from eating just one peanut, the biochemistry of a very small number of people will make them susceptible to getting become hooked on smack after just, as you've indicated, three consecutive uses.
Conversely, when I lived in Brooklyn I had a friend who used (inhaled) small amounts of heroin from time to time. It seemed to have a relaxing, deeply thoughtful effect on him. He did this for at least the fifteen years I lived across the street from him. The last time I saw him, which was another ten or twelve years after I moved away, he hadn't changed much and was still the same healthy, interesting and funny fellow I'd known.
So the conclusion of everything I've observed and learned about addiction over the years holds that susceptibility depends on an individual's biochemistry and psychological predisposition.
Re: the rest of your message: Well said!
You know, I have been helping heroin addicts get clean and productive for a good amount of time now. I work at a place that deals with this problem and I do interventions.
To pinpoint one reason, even one reason that stands out beyond all others is almost impossible.
Let me say this though. Most of those that use heroin were introduced to it while at some place smoking pot. Pot being illegal tends to put people in situations that other illegal drugs are also being used. This brings them into original contact with heroin, and coke, meth etc....
Now, your average beer drinker is not going to end up in these places because there is no fear of being caught with beer.
Its possible that a strong defense against heroin could be as simple as making weed legal. If its brought out in the open, then there is less chance of someone having to find a place to hide to smoke it. Less chance of being introduced to other drugs. The marijuana is not a gateway drug on its own, but it is one of the keys that open the door to exposure to some of those other drugs.
and honestly, someone that does heroin for the first time, and does a small amount, is going to like it. there is no question.