The Composite Operator (CJT) Formalism in the $(\lambda\phi^4+\eta\phi^{6})_{D=3}$ Model at Finite Temperature

We discuss three-dimensional $\lambda\phi^4+\eta\phi^6$ theory in the context of the 1/N expansion at finite temperature. We use the method of the composite operator (CJT) for summing a large sets of Feynman graphs. We analyse the behavior of the thermal square mass and the thermal coupling constant in the low and high temperature limit. The existent of the tricritical point at some temperature is found using this non-pertubative method.