In this paper, we consider the problem of designing mechanical constraints for a planar serial chain formed with three revolute joints, denoted as the 3R chain. Our focus is on the various ways that two RR chains can be used to constrain the links of the 3R chain such that the system has one degree-of-freedom, yet passes through a set of five specified task positions. The result of this synthesis process is a planar six-bar linkage with one degreeof- freedom, and we obtain designs for each of the known Watt and Stephenson sixbar topologies, except the Watt II. We demonstrate the synthesis process with an example.