Is the only difference between a natural minor and major scale really the 3rd note? What about the 7th note? Doesn't the major scale have a major 7th and natural minor scale a minor 7th? Why is this?
Is the only difference between a natural minor and major scale really the 3rd note?
No - in the natural minor, the 3rd, 6th and 7th are all one semitone flatter compared to the major scale (and are therefore a minor third, minor sixth, and minor seventh respectively.)
What about the 7th note? Doesn't the major scale have a major 7th and natural minor scale a minor 7th?
Yes, as above... and again as above, same with the sixth.
Bear in mind that the words 'minor' and 'major' might refer to various different things - not just the major scale and natural minor scale. Someone might reasonably tell you that the main difference between 'major' and 'minor' tonality in general is that 'minor' has a flattened third. There are a number of different 'minor' scales, for example - not just the natural minor.
This may not answer, but if you mean the melodic minor, then yes, the only difference is the b3, compared with the parallel major. That's ascending in classical melodic, or both ways in jazz melodic.
The harmonic minor does have the same leading note as the major, put there specifically to create the semitone interval back to the root, in comparison with the natural minor.
If you mean the natural minor compared with its relative major, there are no differences at all.
There are three variants of the minor scale. Call them three modes if you like.
The Natural Minor is strictly according to the key signature. A natural minor is all natural notes. C natural minor has three flats. And so on.
The Harmonic Minor sharpens the 7th note. A harmonic minor has G#. C harmonic minor had B natural - cancelling out the Bb in the key signature.
The Melodic Minor sharpens the 6th and 7th notes going up, restores them to the key signature coming down. A harmonic minor has F# and G# going up, Gnat and Fnat coming down. C harmonic minor has Anat and Bnat going up, Bb and Ab coming down. (So a Harmonic Minor coming down the scale is the same as the Natural Minor.)
Why? Because Common Practive harmony, the system based around dominants and tonics, makes great use of the leading note - the 7th of the scale with it's strong tendency to resolve to the tonic, particularly when reinforced with a dominant 7th chord. So the Natural Minor was modified to include a leading note. That gave a rather lumpy augmented interval between the 6th and 7th notes of the scale, not nice to sing. Hence the Melodic Minor scale.
There are other scales (modes) with a minor flavour. The defining characteristic is indeed the minor 3rd between first and third notes.