All the above commenters have it right, but I feel like having at least a little bit of understanding how and why to use these scales is more important.
Basically, the melodic and harmonic minor scales have a Major 7th (a B natural if you are in the key of C) for one reason - It is the “leading tone”. I remember telling you about basic chord progressions and the use of the V chord in music to lead back to the I chord.
This movement of chords to the tonic (the I/i chord) is the same in both minor and major keys, except if you are composing in a true modal nature in which case you would just play in the Aeolian or Natural minor the whole time, and not even have a “leading tone” - you would have a minor v going to a minor i.
Back on topic - In modern music (ever since the 16th-ish Century forward) composers have began using Major V (G B D in the key of C) to a minor i (C Eb G in the key of C) in minor key compositions. They did this to achieve the same sense of resolution in the music that was present in Major key compositions (which comes from the resolution of B in the V chord to C in the I chord).
This paradigm shift ushered forth the 3 scales people explained above - Natural (Aeolian), Harmonic, and Melodic. But like I said before it helps more to understand WHY these 3 scales exist and why they shouldn’t really be conceptually different from one another, because if you are writing a complex piece of music in the classical style in a minor key you will quickly begin to realize the blurring of these scales.
Here is a Bach Fugue in C-minor to kind of show you how in about 6 measures he is using Ab, A, Bb and B all in the key of C minor.
Ab is used when Bach is on the ii° iv or VI chord, or as a passing tone on i III, or VII chords. (The ° stands for diminished, or a fully minor chord built upon minor 3rds (D F Ab)).
A is used mainly as a passing tone during a V chord (G) or as a chord tone on the V7/V chord (D7).
Bb is used as a passing tone on i ii° iv and VI chords, and as a chord tone on III and the VII chord.
B is used as the leading tone during the V or vii° chords. The vii° chord is used mainly as a substitute for the V7 chord, as they sound very much alike (the V7 is G B D F - the vii° is B D F)
There is also a chord built upon the Major 6th in a minor key, but I personally haven’t really seen it that much, but just in case you wanted to know it is a vi° (A C Eb) chord.
This might be a lot to digest at once - it took me a really long time to learn it all myself, but this is what I have come to understand about minor key harmony involving the combined usage of the melodic, harmonic, and natural minor scales.
Basically this whole complicated system came out of the need to use a G Major (G B D) in a minor key to go a i minor (C Eb G) - this is why they “invented” the melodic and harmonic minor scales.
Triplets are basically what they sound like - a grouping of 3 notes. Lets say you’re playing a constant stream of quarter notes, and you come across a group of 3 eighth note triplets like in the picture above. You play these 3 notes in the exact same period of time that you would have played a single quarter note.
Another helpful tip in determining how fast to play triplets is this neat system that I learned a while back. Triplets are called “quarter note triplets”, “eighth note triplets” or “half note triplets” etc. The important thing you can remember is that these triplets happen in the same amount of time as their next “bigger” or “longer” note.
Examples of this:
- 3 eighth note triplets happen in the same time as 1 quarter note.
- 3 quarter note triplets happen in the same time as 1 half note.
- 3 half note triplets happen in the same time as 1 whole note.
I hope this helps you out! Sorry if it was too long - I felt that I should be really concise about my explanations.