This paper deals with a special class of three-degree-of-freedom parallel manipulators. First, the singular configurations of the two Jacobian matrices are studied. The isotropic configurations are then found based on the characteristic length of this manipulator. The isoconditioning loci for the Jacobian matrices are plotted to define a global performance index allowing the comparison of the different working modes. The resulting index is compared with the Cartesian workspace surface and the average of the condition number.