You are talking about two different things. In the first paragraph, you are talking about parallel minors, where the root note of both the major and minor scales are the same.
The relative minor, as you have mentioned in the second paragraph, can be found by keeping the same key signature as a major key, but moving the tonic note down a minor third.
When you do this, you will see that between two parallel keys, natural minor has the same configuration as melodic descending minor. From a major scale, you would flat the 3rd, 6th, and 7th.
The rules you have stated appear to be correct, however, understand that only the natural minor form has its own key signature. When writing music in harmonic minor (major scale with flat 3 and 6), you use the key signature of of natural minor (major scale with flat 3, 6, and 7) and then alter the 7th with an accidental to match harmonic minor.
To put it another way, consider the following:
- C major - 0 sharps, 0 flats
- Find the relative minor with the same key signature: A minor
- Compare A minor (0 #/b) to A major (3#s). These keys are parallel.
- the Minor key has 3 fewer sharps than the parallel major key. When comparing keys, three fewer sharps is equivalent to three more flats. For example:
- Come back to C major, and find the parallel minor key (C minor).
- C minor has 3 flats (three more flats than the parallel major key)
- These flats are Bb, Eb, and Ab; which correspond to the 3rd, 6th, and 7th. As we have learned, altering those notes from the major scale brings us to the natural minor scale.
- To write music in C harmonic minor, start with the key signature for C minor (3 flats: Bb, Eb, Ab) and then raise the 7th note back up to a major 7ths with an accidental in the measure. Since Bb is in the key signature, we raise the note with a natural.