Harmony
One can play a chord with three notes using the 1st, 3rd and 5th notes of the scale of whatever key the music is in.  This gives a chord which sounds like the “home chord”.  This means that at least three notes are needed for harmony.  In most homophonic music there are four: for example a choir will normally divide into soprano, alto, tenor and bass, or a string quartet will divide into violin 1, violin 2, viola and cello.