Voltage divider resistor PN balancing
The idea is to have the equivalent of two voltage sources with equal series resistances (Thevenin sources) for each input.

The equivalent circuit of a voltages source combined with a voltage divider consisting of resistors R1 and R2 (that's what you have at the non-inverting input) is a Thevenin source with resistance \$R_{th} = R1 || R2\$.  

I.e. both circuits in following diagram are equivalent:







simulate this circuit &ndash; Schematic created using CircuitLab

So the source at the inverting input should also have a series resistance \$R3 = R1 || R2\$.