“‘That there was something natural, if not also divine,’ remarks Professor Pollard, ‘in the separation of mankind into three classes seemed as clear to mediaeval philosophers as it did to nineteenth-century railway companies’ (and does—he might have added—to some university examining boards). The idea, he reminds us, is as old as Plato; and no doubt it is much older. But we need not investigate the mysterious attraction which the number ‘three’ has always had for the human mind, nor attempt to trace the course of the idea in the Christina Fathers and in the mediaeval philosophers. The immediate point is precisely the feeling of the naturalness or even divinity of the threefold division of society—a division in no respect thought to be a result of royal will” (Chrimes, English Constitutional Ideas in the Fifteenth Century, 94, in his discussion of the three estates).
And did you further know that people are more likely laugh at and remember things when they come in threes? The Romans had a phrase for it: omne trium perfectum (every triad is perfect). Indeed, it seems that we are also more likely to believe things or consider them to be significant when they come in threes. Advertisers, of course, have made hay with this principle; but so have very good storytellers indeed. One cannot sit on a two-legged stool; a three-legged stool satisfies not only the (deplorably duplex) feet, but the mind as well.
That hyperlink is not an endorsement of the website or the ideas contained therein, by the way. Being a Christian, and one moderately sympathetic to Platonism, I have my own suspicions about the importance of the number three. For the moment, I will simply say that, not merely in the notoriously fertile and unreliable imagination, but even in the world of Euclidean arithmetic and geometry, there seems to be something qualitatively different about the number three. One is not really a number, and two is questionably so; three is the first truly number-like number. Two is a company; three is a crowd! (so went the old Shoppers Food Warehouse slogan, which promised to open a new register once the lines got too long). But “crowds” by this definition are comparatively lovely things. One is the number of solitude, and two is the mirroring number: the devil’s number, according to certain mediaeval and Renaissance thinkers, though (and, but of course?) also the number of romance. But “Baby makes three”; or, as it is expressed in that immemorial sacred trifold rhyme dating no doubt back to the caveman, “First comes love, then comes marriage, then comes baby in a baby carriage.”
And card games, naturally, involve sets of three as well.