How often have you heart "The third time's the charm."? Seems those words are around a lot. In plotting, having three main characters can add depth to the story. There's also the bit in a story when the maincharacter tries once and fails, then twice and fails and the third time has success.
By looking at the third time a writer knows to make a story interestingto the reader, the tension must increase. This can happen with a failure to achieve a goal once and then twice. The tension increases and the reader may think two ways. The first is to give up andseek another gol. That will make the story fall flat and into that "so what" pattern. A second failure to achieve ramps up the tension and then the reader wonders if the character will try again. In a good story the character does and succeeds. Having this character fail a third time will really make the story meaningless. There has to be a win somewhere to satisfy the reader. So remember the rule of three. Three characters makes a better story than one or two. The interactions increase. Three tries with the final one succeeding brings a satisfactory ending to the story.