£1.30/litre = £5.90/gallon
Assuming 30mpg, 3 + 3 miles = 6 miles = 1/5 of a gallon
= £5.90 / 5 = £1.18
So if the round trip of 6 miles costs £1.18, and you save 3p / litre
... it is worth driving the extra distance if you put in 118 / 3 = 39 litres.
39 x £1.30 = £50.70
This does not allow for wear and tear on the car.
But your "break even" point, where you cover the extra travel cost, is about £50.
So ...
If you are putting MORE THAN £50 in the car ... go to the CHEAPER station.
If you are putting LESS THAN £50 in the car ... go to the CLOSER station.