Humanoid robots have many potential applications in man-made environments, including performing hazardous tasks, assisting the elderly, and as a replacement for our aging workforce. However, generating a reliable gait for biped robots is challenging, particularly for dynamic gait and in the presence of unknown external disturbances, such as a bump from someone walking by. In this work, a 3D formulation of the Foot Placement Estimator is used with a high-level control strategy to achieve a dynamic gait capable of handling external disturbances. A key benefit of this approach is that the robot is able to respond in real time to external disturbances regardless of whether it is at rest or in motion. This strategy is implemented in simulation to control a 14-DOF lower-body humanoid robot being subjected to unknown external forces, both when at rest and while walking, and shown to generate stabilizing stepping actions.