triad

Short definition from Wiktionary

Noun

1. 1
   A grouping of three.
2. 2
   A word of three syllables.
3. 3
   A branch of a Chinese underground criminal society, mostly based in Hong Kong.
4. 4
   On a CRT display, a group of three neighbouring phosphor dots, coloured green, red, and blue.
5. 5
   A chord consisting of a root tone, the tone two degrees higher, and the tone four degrees higher in a given scale OR any chord with three notes.