While hitting a moving target?
The target, JFK in this case was slowly moving away from the shooters position. Based on my understanding of the events, JFK would have presented Oswald with a target that was slowly shrinking while moving slightly upward and to the right in a straight, predictable line.
Oswald had all the time to make the first shot, remember, the clock starts with his first shot. Oswald fires the first shot, reloads and aims just ahead of where he predicts the target will be and fires when the target moves into his sight picture. He repeats the process for the third shot.