Today, there is not much to differentiate the three brands in terms of prestige. Audi's reputation is equal to the other two. However, in my opinion, where it loses out is in the fact that it is not defined by any one particular parameter.
The BMWs are always fun to drive. Mercs have the best ride quality. Audi is kinda 50-50.
However, in India, the new A4 has street presence like nothing else. No 3 series or C class will look that good on the road.
Audi is ahead of Merc and BMW in China and in some markets in Europe as well.