A ternary ferroelectric ceramic system, (1xy)Pb(In1/2Nb1/2)O3xPb(Zn1/3Nb2/3)O3yPbTiO3 (PINPZNPT, x=0.21, 0.27, 0.36, 0.42), was prepared using a two-step precursor method. The phase structure, dielectric, piezoelectric, and ferroelectric properties of the ternary ceramics were systematically investigated. A morphotropic phase boundary (MPB) was identified by X-ray diffraction. The optimum piezoelectric and electromechanical properties were achieved for a composition close to MPB (0.5PIN0.21PZN0.29PT), where the piezoelectric coefficient d33, planar electromechanical coupling factor kp, and remnant polarization Pr are 660pC/N,72%, and 45C/cm2, respectively. The Curie temperature TC and rhombohedral to tetragonal phase transition temperature TRT were also derived by temperature dependence of dielectric measurements. The strongly bended MPB in the PINPT system was found to be flattened after addition of PZN in the PINPTPZN system. The results demonstrate a possibility of growing ferroelectric single crystals with high electromechanical properties and expanded range of application temperature.