This paper presents a simple Cartesian practical technique named blocked-off-region procedure to study the combined conductive and radiative heat transfer in three-dimensional irregular geometries. By a concept of blocked-off method previously applied in the problems of computational fluid dynamics, both straight and curvilinear boundaries can be treated. The set of equations consisting of gas energy equation and also the radiative transfer equation are solved simultaneously to obtain the temperature and radiative heat flux distributions inside the participating medium. The finite volume method has been adopted to solve the energy equation and the discrete ordinates method (DOM) is used to model the radiative transfer in an absorbing-emitting and linear anisotropic scattering medium. The radiative conductive model is validated by comparison with test cases solutions from the literature and is applied to analyze the effect of thermoradiative parameters such as conduction-radiation parameter, optical thickness and scattering albedo on the temperature and radiative flux distributions for three-dimensional enclosures. Results confirm that the proposed method is a good general way for studying combined conductive and radiative heat transfer in three-dimensional complex enclosures.