The monostable-bistable transition logic element (MOBILE) composed of resonant tunneling diodes (RTDs), is very suitable for threshold-logic circuit design. The three-variable universal threshold-logic gate is designed by MOBILE, which can realize all three-variable threshold-logic functions. But three-variable nonthreshold-logic functions need to be transformed into the form of threshold logic. By transforming the Boolean domain of functions into the spectral domain, three-variable nonthreshold-logic functions are divided into 4 classes. And a new method of three-variable nonthreshold-logic function transformation is proposed according to the extension of the spectrum technology property. The method uses only one two-variable or three-variable XOR transformation. Each kind of the designed circuits using the method, only needs two level circuit series and two basic gates. Through the HSPICE simulation and comparison of power consumption and device number, the circuit not only has the correct logic function, but also can obviously reduce the power consumption and device number.