Easier to use Plane Source
The plane source seems very awkward to use - specifying three corners of the plane. A more natural way to specify a plane is by the origin and normal vector. A radius would also be needed to specify how large the patch of the plane should be. Maybe a "Square Sphere Source" source could be added on top of the existing Sphere Source so people can use the existing one for "difficult" planes (non-rectangular) and the new one for "easy" planes (square).
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Jussi Mattila commented
Another way is to use the dip direction (azimuth of creates dip) of the plane and the dip value (angle between the plane and the horizontal), a method typically employed in geological applications. Similar method could be applied to arrow sources. I also suggest that the origin can be defined by snapping to pre-existing points in the project.