By placing rows of 3rd intervals above minor scales, chords are created in the same manner as when harmonizing major scales. However, because of the different sequences of tones and semitones in the harmonic and melodic minors, some of these chords will be different to those in major keys. Below is an A natural minor scale harmonized as 7th chords. Notice that the chords are exactly the same as those contained in the key of C major. The only difference is the starting and finishing point. Because the minor scale starts on A, A minor will now be chord I instead of V.
If you harmonize the harmonic or melodic minor scale, the chords will not be identical to those of the relative major. Below is the A harmonic minor harmonized as 7th chords. The raised 7th degree results in different chord types for chords I (a minor chord with a major 7th: m/maj7), III (a major 7th chord with a raised 5th: maj7#5), V (a dominant 7th chord) and VII(a diminished 7th chord).
By harmonizing the ascending melodic minor scale, even more of the chords are altered. As you can see from the table below, none of the chords are the same as those derived from the natural minor. The fact that there are three different minor scales gives you many chord options for harmonizing a melody in a minor key.