Well have a read of this and get the software and cable for around a tenner
http://www.audi-sport.net/vb/a3-s3-...-help-please-read-before-posting-updated.html
...or look on the VCDS section and see if there are any willing members near you that would scan the car for you..
VCDS (formerly VAG-COM) forum
...or take it to a garage and the can scan it for you...
Either way there will be a code in the ABS module and it will help you (if not tell you outright) why the light is on...
Guessing will not help and potentially end up costing you more as you replace bits you think is wrong... these cars have control modules all over them and the all store fault diagnostic codes when a fault occurs
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