Voltages from various sources referred to the common ground are added in this circuit. The gain of the output can be controlled by selecting appropriate resistance values. If R1=R2=R3=R=2*Rf then output, Vout=Va+Vb+Vc. By varying the Rf value, gain of the circuit can be controlled.
Op-Amp as Summing Amplifier
This is Non-Inverting summing amplifier. If the common node of resistors is connected to V- terminal then it is called Inverting summing amplifier. The output of inverting amplifier for above mentioned resistor values is, Vout=-(Va+Vb+Vc). The output of Non-Inverting Summing amplifier follows the equation,
Simulation of Op-Amp as Summing Amplifier