The MOnostable-BIstable transition Logic Element(MOBILE) is an important logic unit based on Resonant Tunneling Diode(RTD) circuits, which is very suitable for the design of threshold logic circuit. Threshold circuits such as Threshold Gate(TG), Generalized Threshold Gate(GTG) and etc., can be constructed by MOBILE. In this paper, a novel three-variable XOR gate composed of GTG is designed, through transforming the three-variable XOR function into a more simple and ideal form of GTG input/output function. And a novel full-adder is designed by using the three-variable XOR gate. Through HSPICE simulation and performance comparison, the full-adder has less devices, shorter delay-time, higher operating frequency, lower power consumption and lower power-delay product.