triad vs triumvirate

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. 

triumvirate

noun
  • An official group of three people, especially a ruling council of three men and particularly (historical) two such councils in Roman history. 

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