One of the methods for enhanced oil recovery from heavy oil resources is in-situ combustion. As complicated as this process is, it also has higher efficiency compared to other processes of heavy oil recovery and can be used in a wide range of resources. On the other hand because this trend can be used in deep resources it is appropriate for the resources in Iran. Therefore research and review regarding this process have been significantly increased within the recent years due to international need for oil. One 3D model in three phases of oil, water and gas has been considered for modeling in-situ combustion process. For the components which are distributed in two or more phases, phase equilibrium has been considered among the phases. In this model the energy conservation equation, total and partial mass balance equations have been written for all components including oxygen, non condensable gas, light oil hydrocarbon, heavy oil hydrocarbon, water and coke. Additionally the auxiliary equations are presented for calculations of permeability changes, capillary pressures, reservoir porosity, PVT properties of fluids, viscosity, enthalpy and etc. Finally constraint equations including mole fractions in two phases of oil and gas and also saturation percentage of phases are added to the equations. After writing the equations and their discreteness, the finite difference procedure has been used for solving the equation by applying the boundary and initial conditions. The related program has been written in the Matlab Software. In the next stage accuracy of the present model has been reviewed comparing with experimental data and result of the other models. After assuring the accuracy of the model, the present model was used in one of the heavy oil reservoir for reviewing the in-situ combustion process and the results were gathered in the conclusion section. The reservoir simulation results in 1D mode shows that almost 90% of the reservoir in-situ oil has been recovered after 150 days of simulation from which 30% is related to production of light oil due to cracking reaction. At the same time in such condition recovery rate of water is over 100% which is due to water production as the result of oxidation reactions. These results show that in 3D modeling of the reservoir location of the injection well is seriously effective on parameters like oil and water saturation, and the blocks close to the injection well show faster reactions toward air injection. The process simulation in such condition shows that after 150 days after initiation of the simulation by passing the combustion front through the reservoir blocks, the temperature in theses blocks will be maximized which is 870 Fahrenheit is the maximum. Keywords: In-situ combustion; Heavy oil; Enhanced oil recovery; Modeling; Reservoir