All bad things happen in threes
Again this has to do with such things as the holy trinity, the triple Goddess, and other religious symbols. It was present even in ancient neolithic times in the form of symbols such as the triple spiral. However some sources come from notes on a pattern, such as soldiers learning in the trench if they shared a match for three cigarettes the third person would likely die as the enemy would have locked in on the flame by then. Or there is also the alleged 'rule of three', which is an American based superstition where celebrity deaths tend to occur in threes.