Shown below is an A natural minor scale harmonized as 7th chords. Remember that the chords will be exactly the same as those contained in the key of C major (the relative major key). The only difference is the starting and finishing point- Because the minor scale starts on A, Am7 will now be chord I instead of VI.
If you harmonize the harmonic or melodic minor scale, the chords will not be identical to those of the relative major. Here 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. Now 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.
As mentioned earlier, it is common to find chords derived from all three minor scales in one progression. This example uses minor key scale tone 7ths.