The electronic transport sensitivity depending on the coupling interface between a phenalenyl molecule and atomic chain lead in three-electrode single-molecular device is theoretically modeled and studied using self-consistent tight-binding method. Electronic transmission probabilities are obtained by changing the hopping integral on the interface. The result shows that the electron transmission probability lowers as the coupling strength becomes weak. Not only the transmission probability shows high peaks, but also the transmission energy bands become wider as the coupling strength increases. A novel property of the symmetrical three-electrode phenalenyl molecule device acting as a good plus-minus energy switch without source is discovered.