The traditional approaches for evaluating the stability of slopes or earth dams subjected to water drawdown are performed under plane-strain two-dimensional (2D) condition. Three-dimensional (3D) effect is neglected in assessment of the safety of a slope limited by rigid structures or a dam constrained by a narrow valley. Based on the kinematic approach of limit analysis, a 3D rotational failure mechanism is adopted here to investigate the influence of water drawdown on stability of 3D slopes. Several stability charts are presented to conveniently estimate the safety factor of 3D slopes under four different types of drawdown processes. An example is given to demonstrate the difference in the safety factors obtained from 2D and 3D analyses. When a slope is constrained to a large width (the ratio of the width to the height B/H ≥ 10.0), the 3D effect can be neglected and the plane-strain analysis is appropriate to assess its safety.