Lens system configurations of the close-close (left panel) and close-wide (right panel) 3L1L models. The inset in each panel shows the locations of the three lens components, denoted by blue dots labeled M₁, M₂, and M₃. The dashed circle in the inset represents the Einstein ring. The line with an arrow represents the source trajectory. The three small empty dots on the source trajectory represent the source positions at the epochs of t₁, t₂, and t₃ that are marked in Fig. 1. The size of the dots is not scaled to the source size. The red figure represents the caustic, and the gray curves around the caustic represent equi-magnification contours.