Planar 2-degree-of-freedom (DOF) 3-differential Cable-Driven Parallel Robots (CDPRs) consist of a point-mass end-effector driven by a number of cables. Each cable is divided into four segments, three of them being connected to the point-mass end-effector by means of routing pulleys. This paper deals with the stiffness analysis of such planar 2-DOF 3-differential CDPRs. Based on the usual linear spring cable elongation model, the expression of the stiffness matrix is derived. The stiffness and workspace of several examples of planar 2-DOF 3-differential CDPRs are then compared. The results of these comparisons illustrate that the stiffness of planar CDPRs can be significantly improved by means of pulley differentials.