Non-inverting input terminal is grounded. Voltage V1, V2 and V3 are feed into inverting terminal. Resistance R1, R2 and R3 are used as feedback resistors. As no input current flows into the amplifier, the sum of the input currents I1, I2 and I3 must be equal to the current I flowing to the output terminal.
The output current for summing amplifier is given by
V0 = -R0(V1\R1 + V2\R2+ V3\R3)