A three note chord is a triad. In root position, a 5-limit triad spans a perfect fifth . The fifth is divided into a major and minor third . If the lowest two notes are separated by a major third, you have a major triad . This is called octave invariance of harmony .
Some people take it very seriously.
Midi files are provided of major and minor triads. Along with the thirds and perfect fifth previously mentioned, these are 5-limit consonances . A chord is a 5-limit cosonance if it contains no other intervals than these, or octave equivalences. In traditional harmony, only the 5-limit is recognised, so such chords are simply defined as consonances. Other chords are dissonances.