  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.
