Where there’s only one type of MAJOR scale, there are 3 primary types of minor scales. Each with its own distinctive sound.
NATURAL MINOR SCALES
To see the influence of different intervals at the beginning of a scale, let’s take a look at the A minor scale. This scale was chosen because it’s the relative minor of the C MAJOR scale (more about this can be found in the section on Modes). Now, if you guessed that a minor 3rd (m3) between the TONIC and the MEDIANT indicates a minor scale – you’d be exactly right! (2 + 1 = 3) So, any time you see this INTERVAL pattern at the beginning of a scale or mode, you should think minor.
The HARMONIC minor raises the VII (7th) degree by a single half-tone creating a minor 3rd (m3) between VI and VII. Also of note is the change from a SUB-TONIC to a LEADINGTONE between the VII and i scale degrees. It becomes a LEADINGTONE because the interval was reduced from 2 half-tones (G to a) to a single half-tone (G# to a).
The MELODIC minor raises the VI (6th) and VII (7th) degrees by a single half-tone. You’ll see different sources referring to this as the “ascending” MELODIC scale which hints at the fact that they expect the “descending” scale to be something different – and they do. In this case they expect the NATURAL minor to be played while descending. I say use whatever works for you.