This paper presents a new approach for the global electromagnetic analysis of the three-Terminal active linear and nonlinear microwave circuits using the Finite-Difference Time Domain (FDTD) Method. Here, we have updated the both electric field components on the three - terminal active device by correlating the voltage and current with its impedance. This approach is applied to the analysis of a linear amplifier which includes a three-terminal active MESFET device. Simulations results are in good agreement with those of the commercial tool.