Looking at the Audi web site the kerbweight of the 2 litre TDi (140 bhp) is about 1350 kg. Applying the 85% (sensible) guideline the MTPLM figure works out at 1150 kg. My advice would be to ignore Audi's 1700 kg max tow limit. No way could an A3 safely tow 1700 kg!
Towing a van of MTPLM 1275 kg...