'Trouble comes in threes' or 'bad luck comes in threes' is a concept often used in literature that is common in Western culture.
Since natural things always come in pairs (think of your eyes, hands, feet, ears), anything that comes in threes is often associated with the abnormal, unnatural or troublesome.
This concept is clearly seen in the short story "The Monkey's Paw" by W. W. Jacobs.
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