"Death comes in threes" origin?

Solution 1:

The fact that the number "three" is a source of superstition is based on a number of different reasons and is generally referred to as "Triaphilia". I think that "death comes in three" is just a superstition based on the same concepts:

• The tendency to want to hold on to the three connection is strong in many areas of life.

  • One reason might be a sort of number mysticism. Three is the first odd prime number, the triangle is a stable shape, in our base 10 system, the fraction 1/3 is .3333333…, et cetera.

  • A second more compelling reason might be psychological, perhaps deriving from the structure and limited complexity of our brains.

  • The appeal of the trinity in Christianity and other religions, the philosophical triad of thesis, antithesis and synthesis, and even the setup of many jokes seem to stem in part from a natural resonance with the number three. (A priest, a minister and a rabbi go into a bar and ..., or a physicist, an engineer and a mathematician are asked how to … .)

(abcnews.go.com)

There are a few early instances of the expression in the 30's but usage appears to be more frequent from the mid 60's, Ngram. It might be just a variant of the more common "bad things come in threes", Ngram.

Solution 2:

We tend to like threes.

Take, for example, hendiatris. Wikipedia has some wonderful examples of this, e.g.:

• Veni, vidi, vici
• Liberté, Égalité, Fraternité (liberty, equality, fraternity)
• Friends, Romans, countrymen, lend me your ears
• Sex, Drugs, and Rock & Roll
• Life, Liberty, and the pursuit of Happiness
• Location, location, location

Threes seem somehow natural and right in rhetoric.

Like Josh61 I would suggest the idea that death comes in threes is a mere outgrowth of this psychological liking for threes.

Solution 3:

There is some math behind the idea that celebrities die in threes - although it turns out the number is actually 2.7183, or e.