In this article, a three-dimensional thermal and hydrodynamic numerical model using a flat heat pipe (vapor chamber) is proposed for cooling of electronic components. The heat pipe is composed of a vapor region sandwiched between two wick regions, which are covered by two flat copper plates. A three-dimensional hydrodynamic model was developed to solve the fluid flow through the liquid and vapor regions. The hydrodynamic model is coupled with a three-dimensional thermal model using the energy equation to estimate the model temperature. The hydrodynamic model takes into consideration the return liquid between the two wick regions. An implicit finite-difference method is used to solve the theoretical model. The numerical model was validated by using an experimental work and the results of the literature. Good agreement was found between the numerical results and the experimental work and the literature. The effect of the power input and heat transfer coefficient of the cooling fluid on the performance of the vapor chamber was studied. Our model's results illustrate well the movement of the working fluid in the wick and vapor regions. They also indicate that when the power input or the heat transfer coefficient of the cooling fluid increases, the maximum pressure difference between the heat pipe and convection inside the wick regions increases.