A ternary ferroelectric ceramic system, (1−x−y)Pb(In1/2Nb1/2)O3–xPb(Zn1/3Nb2/3)O3–yPbTiO3 (PIN–PZN–PT, 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 (0.5PIN–0.21PZN–0.29PT), where the piezoelectric coefficient d33, planar electromechanical coupling factor kp, and remnant polarization Pr are 660 pC/N,72%, and 45 μC/cm2, respectively. The Curie temperature TC and rhombohedral to tetragonal phase transition temperature TR−T were also derived by temperature dependence of dielectric measurements. The strongly “bended” MPB in the PIN–PT system was found to be “flattened” after addition of PZN in the PIN–PT–PZN system. The results demonstrate a possibility of growing ferroelectric single crystals with high electromechanical properties and expanded range of application temperature.