The essence of a number of beliefs is that certain events tend to happen within some (unspecified) period of time. The belief that things “happen in threes” is a perfect example: Many people believe that events like plane crashes, serial-killing sprees, or birth announcements tend to occur in triplets. It is almost certainly the case, however, that these beliefs are mere superstitions that stem from the tendency to allow the occurrence of the third event in the triplet to define the period of time that constitutes their “happening together.”
