triad vs troilism

triad

noun
  • A grouping of three. 

  • on a CRT display, a group of three neighbouring phosphor dots, coloured green, red, and blue. 

  • A chord consisting of a root tone, the tone two degrees higher, and the tone four degrees higher in a given scale. 

  • A branch of a Chinese underground criminal society, mostly based in Hong Kong. 

  • A word of three syllables. 

troilism

noun
  • The practice of three people having sex, i.e. having threesomes. 

  • The practice of two people having sex while a third person watches (and may or may not take part). 

How often have the words triad and troilism occurred in a corpus of books? (source: Google Ngram Viewer )