Try not to think of (different) minor scales but of one minor key in which the sixth and seventh notes are variables. - They can be according to key signature or raised by one semitone (half-step).
So in A minor, the available notes are A,B,C,D,E,F,F#,G,G#.
Any of these notes can be found in triads within the key of A minor. That means that D minor (D-F-A) and D major (D-F#-A) are both possibilities for example.
In practice, some of the triads are much more common than others, and some are extremely rare (occurring in music of the Common Practice Period only in very specific circumstances). This is true of the major key too of course; the diminished triad on vii for example is rare in root position.
So, some general rules of thumb to start with:
Augmented triads are seldom used. This means for example that C-E-G#, while theoretically available in A minor, is best avoided.
Diminished triads in root position are also rare, although they may be used in first inversion.
In the minor key, the raised seventh - the "leading note" (G# in A minor) is normally best followed by the tonic.
The raised sixth (F# in A minor) is normally best followed by the raised seventh.
Within a part, augmented intervals are best avoided and the leap of a diminished interval should be followed by notes within that interval. In minor keys, the raised sixth and raised seventh need to be treated carefully with regards to this.
A cadence will be most satisfactory when the dominant chord is major (where applicable). So for example use E major (E-G#-B) in A minor.
For more information, consult a good harmony book such as the one below.
Check out this textbook on
Four Part Harmony.