code: public static double angleBetween3D( Coordinate c1, Coordinate c2, Coordinate c3 ) {
          double a = distance3d(c2, c1, null);
          double b = distance3d(c2, c3, null);
          double c = distance3d(c1, c3, null);
  
          double angleInTriangle = getAngleInTriangle(a, b, c);
          double degrees = toDegrees(angleInTriangle);
          return degrees;
      }
docs: Calculates the angle in degrees between 3 3D coordinates.
  
  <p>The calculated angle is the one placed in vertex c2.</p>
  
  @param c1 first 3D point.
  @param c2 central 3D point.
  @param c3 last 3D point.
  @return the angle between the coordinates in degrees.