D major has a key signature of F sharp and C sharp
D minor has a key signature of B flat
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A Dorian key signature is an earlier style of key signature used on pieces in minor keys. A good example is J.S. Bach's Toccata and Fugue in D Minor, BWV 538, that was notated with a key signature of zero flats rather than the one flat of a contemporary key signature notation for D Minor. The Dorian mode is a mode of the major scale built off of the second scale degree of a major scale and, therefore, a Dorian key signature for D Minor would be the signature for the major scale a whole step below: C Major...therefore zero flats. The D Dorian scale is spelled D, E, F, G, A, B, C and therefore shares the same key signature as C Major in this older notational system. It is now customary within the western tonal system to relate minor keys to the key signature of the major key found a minor third above. D Minor is now written with one flat, the key signature of F Major. Today the "natural minor" scale or Aeolian mode (the mode built off the 6th scale degree of major) is the common reference point for a minor key's key signature.
There is not three notes in any scale or key, if you are asking about the accidentals, there is one flat (Bb) in the key signature plus an additional C sharp for the harmonic minor scale.
There is no key signature for D harmonic minor in particular - as the raised seventh is not part of the key signature. In all clefs, D minor has one flat and in all cases (since the raised seventh in this scale is C♯), to avoid confusion it is best to avoid having both a sharp and a flat in the same key signature.
The key signature indicates which notes are sharp, natural, or flat. Every key has a specific signature.
D minor - more specifically, D harmonic minor (the version of the minor scale with a flat 6 and a sharp 7). However, you will never see a key signature with a B flat and a C sharp. The key signature will only contain B flat.