It is a common belief that bad luck, especially relating to death, comes in threes. For instance, whenever two celebrities or public figures pass away, superstitious people expect a third death to occur relatively soon after. In truth, the superstition is simply a form of confirmation bias.
Since the superstition simply states that death or bad events come in threes, and it specifies no particular time frame, any calamity can be considered one of the three qualifying events. If two public deaths occur, the only way the belief would not be "justified" is if there were never another death of a celebrity. Since death and ill luck are natural parts of life, there are always three deaths or other calamities serving as supposed confirmation for superstitious believers.