Research on legged robots has been motivated by its ability of locomotion in rough environments. Planar three-link biped robots are a class of under actuated robots which consists of two knee-less legs and a torso joined at the hip. Controlling an under actuated mechanical system is an extremely challenging problem. This paper focuses on implementing a PD controller to a planar three-link biped robot that has been decoupled and re-structure to form a simple mechanical system using feedback regularization. This notion of feedback regularization is a novel concept that has been previously implemented on a class of hoop robots and spherical robots. The implementation of a feedback regularization based PD controller for the class of planar three link biped robots in this paper is an indication that this notion can be extended achieve stability of many other under actuated mechanical systems as well. By implementing such a PD controller for the robot, it is shown in this paper that the controller can be achieved to be robust to even large parameter deviations.