The three shots that Oswald fired would have been incredibly hard to aim for even a professional shooter because the car was so far away and at such a bad angle, yet Oswald was just the average marksman. Furthermore, he managed to fire off 3 rounds in about 5 seconds. The gun he used was a bolt-action rifle which takes a minimum of 2.3 seconds to bolt. This would've given him no time to aim at a moving target far away.
Zapruder filmed the entire assassination which showed Kennedy being shot multiple times. Oswald allegedly shot from behind Kennedy, however, when Kennedy was shot in the head, his head moved back and to the left which suggests that someone else from the Grassy Knoll shot him.
View Zapruder had while filming
The official Warren Report states that three shots were fired which means that one bullet had to have caused seven bullet wounds. This is called the magic bullet theory.