[en] Dynamical spontaneous breaking of some discrete symmetries including special parities and time reversal and their restoration at finite temperature T are researched by means of the real-time thermal field theory in 3D Gross-Neveu model. When Λ/m(0), the ratio between the momentum cut-off and the dynamical fermion mass at T=0 is very large, the equation of critical chemical potential μ_c and critical temperature T_c will be Λ-independent and identical to the one obtained by auxiliary scalar field approach in 1/N expansion. The dynamical fermion mass m≡m(T,μ), as the order parameter of symmetry breakings, is indicated to have the same (T_c-T)^1/2 behavior at T < or approx. T_c as the one in 4D NJL-model and it shows the second-order phase transition feature of the symmetry restoration at T>T_c. It is also proven that no scalar bound state could exist in this model. (author)