Angle between two positions on hex grid?

Assume that you must use only the Vector3Ints that represent hex-cell positions as input data; for the purposes of this question, world transforms don't exist.

Grid is configured as follows:

• Cell size 1,1,0

• Cell Gap 0,0,0

• Cell Layout Hexagon

• Cell Swizzle YXZ

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Tilemap is configured as follows:

• Tile Anchor 0,0,0

• Orientation XY

• Offset 0,0,0

• Rotation 0,0,0

• Scale 1,1,1

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Given a position A on the tilemap, a position B on the tilemap, and a position C on the tilemap, find the angle between B and C using A as the origin. A, B, and C are all Vector3Ints representing cell positions.

Note that I am aware of the CellToWorld method of the Tilemap that can be used to turn everything into world space and then just find angles the usual way. As noted in the first paragraph, this is not what the question is asking!

Thank you for any assistance.