Complex sentence: Major-thirds tunings are unconventional open tunings, in which the open strings form an augmented triad; in M3 tunings, the "augmented" fifth replaces the "perfect" fifth of the major triad, which is used in conventional open-tunings.
Split into simpler sentences: Major-thirds tunings are unconventional open tunings, in which the open strings form an augmented triad. In M3 tunings, the "augmented" fifth replaces the "perfect" fifth of the major triad, which is used in conventional open-tunings.