Lens system configurations of the four 3L1S models in the population A. For each model, the left panel shows the caustic (red cuspy figure) and source trajectory (line with an arrow) in the central magnification region, and the right panel shows the positions of the lens components (marked by blue filled dots) around the Einstein ring (dotted circle). The sizes of the blue dots are set according to the order of masses of the lens components. Lengths are scaled to the Einstein radius corresponding to the total mass of the lens. The grey curves encompassing the caustic represent the equi-magnification contours.