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).
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.