Offset voltage is a voltage that can be considered to be an independent voltage source that is in series with either the inverting or the non-inverting input of the op-amp itself. It is not applied to the (left) end of R1. There is a difference.
Put a voltage source in series with either input calculate the output voltage assuming an otherwise ideal op-amp. You will find the magnitude of the gain is the same either way (and it is not R2/R1 or -R2/R1).
R3 has no effect on the gain (at least in the context of this simple question). Its only purpose is to equalize the resistances looking out of the op-amp inputs so that input bias currents do not cause an addition error term (ideally).