opamp Vos bias resistor
A simple inverting follower (Gain of "-1") needs two equal resistors in the feedback path.

It is the non-inverting follower (gain "+1") which needs no resistors for 100% feedback.

The resistor R3 is - in most cases - not needed. The only purpose of this resistor (R3) is "offset compensation". Both input nodes draw a small DC current and this small current will produce a small DC voltage (which then will act as an input offset voltage Vo.)

This unwanted input voltage Vo will cause an unwanted DC output. But note that the input offset is multiplied with the noise gain (1+R2/R1) only - and not with the much larger open-loop gain.

This effect can be - up to a certain degree - compensated if app. the same DC input voltage is created at the other (pos.) input. For this purpose, R3 is added with a value of app. R1||R2.

If this compensation is necessary or not depends on the specific application.