This chapter investigates the impact of complex geometries on three-dimensional (3D) slope stability under short-term heavy rainfall based on numerical simulations. Different groups of standard slope models were designed to examine the effects of surface undulation and slope gradient on stability of 3D slopes. For a 4 m surface undulation increment, the 3D slope exhibits lower safety factors than the selected two-dimensional (2D) critical section, indicating that 2D analyses are not necessarily more conservative when surface undulation is sufficiently large. Further analyses show that higher undulation frequency or degree further reduces 3D safety factors, with differences from 2D results becoming pronounced when the undulation exceeds 10 m. These findings highlight the limitations of traditional 2D critical section methods in capturing the effects of complex 3D geometries. Overall, the study demonstrates that 3D slope geometry significantly influences stability calculations under rainfall conditions. Performing actual 3D analyses is therefore essential for slopes with complex geometry. It is strongly recommended that engineers integrate both 2D and 3D assessments to ensure reliable and efficient slope design under short-term heavy rainfall.