We had an interesting discussion last night about why lists of 3 just work! We thought it might have its roots in religion: "Father, Son, Holy Spirit", "Abraham, Isaac, Jacob", etc. Perhaps, it was the pleasing rhythm of X, Y and Z (4 beats including the word "and"). Scrawling the Internet, it seems to be a mathematical reason. Apparently, 3 items is the shortest amount that one needs for a pattern, thus making patterns of 3 the most memorable of patterns. This blog post gives some good help: http://www.copyblogger.com/rule-of-three/