The rule of three for characters: each plotline should have three central characters. This allows for six relationships to be developed, plenty for a novel. Picture a triangle.
The rule of three for plot development: Things usually happen in threes in a story. The first time something happens in a plot, it's an incident. The second time it happens it becomes a pattern. By the third time, the main character must recognize the pattern, learn from it, and do something to change it or risk being TSTL.