The output voltage is defined as Vo= - Gain_factor(V1+V1+...+Vn). For this case, Vo = -10(0.1+0.1+0.1) = 3 Volts. This circuit shows how the output signal is a combinations of the input signals. You may turn ON and OFF each signal individually. With only one signal input, the output will be 1V, with two or three signals, the output will be 2V and 3V, respectively. This circuit model is typically referee as a mixer.
