This paper presents a mathematical development of the explicit finite difference (FD) method approach in modeling the temperature or heat distribution in a three dimensional (3D) problem. Although numerical solutions are only an approximation to the exact solution, the finite difference method can provide solutions is a discrete set of points, further simplified by a system of simultaneous equations ideally suited to computers. This has provides insights into complex problems in the heat transfer analysis. A typical sample used in this study is a single crystal lattice of silver (Ag{100}) with initial and boundary conditions applied at its sides. Using these explicit equations for iterations, the FD simulations with infinite time-steps and a small tolerance, produce as close the approximated results as expected.