Definition of "triad"

There are, says the Irish triad, 'three fewnesses that are better than plenty: a fewness of fine words; a fewness of cows on grass; a fewness of good friends around good ale'. As an Ulsterman I would agree.

2000, David Pierce, Irish Writing in the Twentieth Century: A Reader, page 625

In his general deportment he was pompous and important, affecting a species of florid elocution, which often became ridiculous from his misarranging the triads and quaternions with which he loaded his sentences.

1815, Sir Walter Scott, chapter 13, in Guy Mannering

The point — and the symbolism — was that so long as the informer kept his mouth shut he was alive. Harry had seen the result of zjuk carried out by the Tapei Triad on a poor jerk they found in a back street of Tan-shui. They had used broad nail heads that didn’t make such big holes on their way in. When the paramedics came and pulled the brick off the dead man, the face came with it.

2012 , Jo Nesbø, translated by Don Bartlett, Gjenferd [Phantom]‎ (Fiction), New York: Alfred A. Knopf, page 99