The phase transition and domain configurations in emerging ternary Mn-doped Pb(In1/2Nb1/2)O-3-Pb(Mg1/3Nb2/3)O-3-PbTiO3 ferroelectric crystals have been investigated by high-resolution X-ray diffraction. Phase transition sequences of cubicrhombohedral and monoclinic Atetragonalcubic were determined under zero-field-cooling and zero-field-heating after poling, respectively. Most importantly, a monoclinic-like heterogeneous domain configuration was observed in the conventional high-symmetry tetragonal phase field due to the coexistence of multiscale tetragonal variants. Our results provide further evidence that superlattices with low-symmetry can be assisted by tetragonal nano-twins.