A minor 3rd is an interval consisted of 3 semitones and a Major 3rd is an interval consisted of 4 semitones. These intervals have a very important role in making chords. If minor 3rd is used, it becomes a minor chord, and if Major 3rd is used, it becomes a major chord. These are good guides when practicing chords, so it would be good to train well. Try singing along and then practicing.