One John Dunstable
Was the first to use the third:
Composer John Dunstable (sometimes spelled Dunstaple), 1390-1453, had a great influence on music beyond his native England. Before Dunstable, when two voices sang in harmony, the notes were separated by the intervals of a fourth (the first two notes of Here Comes the Bride) or a fifth (the first two notes of the Theme from Star Wars) or a octave (the first two notes of Somewhere Over the Rainbow) because these were considered “perfect.” For a graphical explanation of intervals, go here to musictheory.net. These intervals give early music its distinctive sound. Here is an example of a chant with parallel movement of notes a fourth apart.
Dunstable got the idea of using not only the interval of a third (the first two notes of When the Saints Go Marching In, major; or Greensleeves, minor), but groups of three notes (a triad), forming chords. The third is considered imperfect because it can make a chord major or minor, giving it a different feel, happy or sad. When we hear Dunstable’s music today, it sounds more familiar than ancient chant because our ears have become accustomed to major and minor chords.