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A reactionless mechanism is one which does not exert any reaction force or moment on its base at all times, for arbitrary trajectories of the mechanism. This paper addresses the synthesis and design of planar and spatial reactionless three-degree-of-freedom (DOF) mechanisms using four-bar linkages without separate counter-rotations. Based on the conditions of dynamic balancing of single planar four-bar linkages developed previously, the suitable architecture and parameters are first determined, and an optimized four-bar mechanism is then designed and prototyped. Planar and spatial 3-DOF reactionless mechanisms are synthesized using the reactionless four-bar linkages. The optimization of the 2-DOF mechanism as the basis mechanism of 3-DOF mechanisms is performed in order to reduce the mass of the mechanism for a given platform. Prototypes of the reactionless planar and spatial mechanisms are designed. With the help of a dynamic simulation software, it is shown that the mechanisms are reactionless for arbitrary trajectories.