As companion animals for humans for thousands of years, cats play all sorts of mythological roles. In ancient Egypt, cats were revered; today, Americans collectively keep more than 81 million cats as pets. Most likely, this superstition arises from old beliefs in witches and their animal familiars, which were often said to take the form of domestic animals like cats.
The belief that bad luck comes in threes is a classic example. A couple things go wrong, and believers may start to look for the next bit of bad luck. A lost shoe might be forgotten one day, but seen as the third in a series of bad breaks the next.
