Many plots have three main events: an inciting incident, a climax, and a falling action (or response to the climax).
The Rule of Threes also applies to story structures. Many stories have, at most, three main characters. Many plots have three main events: an inciting incident, a climax, and a falling action (or response to the climax). If a novel has sections, it often has three sections; if a style has multiple settings, it usually has three main ones.