Lens-system configuration of the 3L1S model. The main panel shows the central anomaly region, and the inset displays the whole view of the lens system, where blue dots marked by M₁, M₂, and M₃ denote the positions of the individual lens components. Gray curves around the caustic represent the equi-magnification contours. The dotted circle centered at the origin of the coordinates represents the Einstein ring of the lens system.