M3AlX (M=Ti/Zr/Hf, X=C/N) compounds are promising high-temperature structural ceramics. However, their interesting polymorphism, thermomechanical stabilities, and thermal and mechanical properties were not fully understood. In this work, the polymorphisms of M(3)AX phases are investigated by combining first-principles and lattice dynamics calculations. Only Ti3AlN shows polymorphic transition between the cubic and orthorhombic phases at around 1105K; but other M3AlX phases do not display similar polymorphic phase transition. Furthermore, the temperature-dependent heat capacity, thermal expansion, and elastic stiffness of Ti3AlN polymorphic phases are reported for the first time to explore the relationship between crystal structures, and mechanical and thermal properties. Ti3AlN polymorphs show anisotropic thermal expansion and elastic stiffness; and the orthorhombic Ti3AlN is suggested as a promising damage tolerant nitride, which has similar properties with the previously reported Zr3AlN and Hf3AlN.