Describes a parabola in 3D space compute(p1,p2,p3) The three points must lie on a plane parallel to xy plane and must not be collinear The focal distance is the distance between the apex and the focus of the parabola. The focus is on the positive side of the 'X Axis' of the local coordinate system of the parabola. Compute the parameter of this parabola which is the distance between its focus and its directrix. This distance is twice the focal length.