I had this on my old Ibiza, not sure if the S3 has the same bulbs but worth a check. Anyway, the bulbs in the Ibiza had double filaments - one filament runs the normal tail lights; the other (brighter) filament runs the brake lights. There should, in theory, only be one way of fitting the bulb into the holder so that everything works hunky-dory.
Anyway, I managed to get my Ibiza bulb in the wrong way (i.e. by 180 degrees) and the net result was the brake light filament was permanently lit rather than the sidelight. Sorted by spinning the bulb back round through 180 degrees.