A3 has a 50 litre fuel tank. Quattro A3/S3 have 55 litre tanks.
0 indication comes on with 1 gallon left in the tank, just like every other Audi. The fuel computer does not calculate range on that last gallon.
Best I've managed in a 1.6TDI is 635m from a tank. Zero come on at 606m and I done a further 29m before filling up with 48.3l back into the 50l tank.
This was 8 x 80m journeys, three adults, 90% motorway on cruise at 70. If it had just been me and an uninterrupted trip, I think the car would stretch closer to 700m from the tank.
In contrast, the best I managed in the S3 was 430m from 54 litres same type of trip, 3 up. That was around 20m after the 0.