Vout =Vin* (R1+R2)/R1
Vout =Vin* (1+R2/R1)
Gain = (R1+R2)/R1
Gain = 1+R2/R1
Canceling effect of input bias current
If you want to be precise, R3 must be equal to R1||R2 . Or R3=(R1*R2)/(R1+R2).
If gain is quite high (10+) then you can use R3 = R1 because R2 will be too high to make significant difference.