In the attached file, the curve is a result of the intersection between the horizontal plane and the 3 vertical surfaces. The straight lines were then scaled1d to extend them past the edges of the horizontal plane.
Use these curves to trim the horizontal plane. When you then ask for the intersection (IntersectTwoSets) between them, only a tiny segment is found.
Yep, I see - thanks. If I mske the plane a little smaller and trim with the red surfaces, I get a bit more back from the intersections - one of verticals and the fillety thing do not return any intersection at all.