People like things in groups of threes.
Think: Goldilocks and the Three Bears, Liberte, Egalite, Fraternite, The Three Musketeers, The Three Little Pigs, Three Billy Goats Gruff and Rock, Paper, Scissors. Past, present, future. Beginning, middle, end.
Three is powerful stuff.
In literature, it’s called the rule of three and in design, the rule of thirds.
Why is three so powerful? The rule of three simply means that people can remember three pieces of information really well in short-term memory; add more items and retention falls off considerably. As more and more items are added to a list, the average person retains less and less. Four items are a bit harder to remember than three. Five items are even harder. Once the number of items on a list hits eight, most people have little chance of remembering the entire sequence.
So, if you’re writing, developing a strategy or designing a layout, keep the power of three in mind.