Nowadays, this choral prelude on "Vom Himmel hoch" (BWV Anh. II 64) is no longer attributed to Johann Sebastian Bach, although the unknown composer uses rising and falling scales, as Bach uses them in his own choral preludes, whose text is related to angels; for example, in Bach's "Canonische Variationen" (BWV 769/a) or in the „Orgelbüchlein“ (BWV 606).
The execution of the composition BWV Anh. II 64 is not strictly three-parted: The texture is occasionally filled up with chords in the second part, which also creates some parallels.
My own Trio-version would not like to „improve“ the original, but offers an alternative with strictly three-part voice guidance.
As download, both versions are ready for comparison.