Mnemonics, up and down
Going up: When the Saints Go Marching In opens with a major 3rd, the "Oh" to "when" leap. Kumbaya makes the same jump on its first two syllables, and Hush, Little Baby on "Hush" to "lit-". Three folk tunes, one interval; pick the one already in your head.
Going down: the most famous four notes in classical music. Beethoven's Fifth hammers one pitch three times and then drops a major 3rd, G down to E♭. Swing Low, Sweet Chariot falls the same distance on its first two words. The fall doesn't sound like the climb, and the same four semitones can sit in different places in a key, so the buttons above play it up, down, and stacked.
Jazz keeps its own reference tunes. Mark Levine's interval table fixes the ascending major 3rd to the opening of Monk's "Monk's Dream" and the descending one to Coltrane's "Giant Steps."
On the staff
The interval that decides the chord
Stack a chord and the root and 5th mostly stay put; the 3rd is the note that changes. Major triads carry a major 3rd, minor triads a minor one. Major 7 and dominant 7 chords sit on a major 3rd; minor 7 and half-diminished sit on the minor. Hear that one distinction reliably and half of chord ear training is already done.
Theory files thirds and sixths together as the imperfect consonances: warm and blending where the perfect intervals are open and hollow, with more pull toward motion. That warmth is why parallel thirds are the default sound of two voices singing together, from Mozart duets to country harmonies, and it's the texture to listen for when the two notes ring at once.
What it gets confused with
The minor 3rd, one semitone narrower, is the real one. Same size on paper, different quality, and the pair gets confused for exactly the reason it matters: it carries the whole major-versus-minor distinction. The imagined-chord test settles it. Continue the interval up into a triad in your head; bright means major, heavy means minor.
The perfect 4th, one semitone wider, trips people early on too. The tell is restfulness. The major 3rd sounds like it has arrived; the 4th leans toward a resolution it hasn't gotten yet. Hold the second note an extra beat: if it sits comfortably you heard a 3rd, and if it itches to move, a 4th.
How Aubel handles it
The major 3rd is the first interval to unlock beyond the starting set (unison, 4th, 5th, octave). It arrives once every starter is mastered: ten lifetime correct answers each, holding seventy percent accuracy over recent attempts. Newly unlocked intervals are oversampled for their first fifteen or so attempts, enough exposure for the sound to settle before it rotates in normally. The detail screen leads with When the Saints Go Marching In.