Well it's a known fact that BMWs are more sporty than Mercedes. Mercedes are more of a luxury car brand that older folks tend to prefer more. And Audi's are higher maintenance than BMW's. I know this from experience. I'v owned numerous BMW's and tried an Audi once and had much more problems. Also my foreign car mechanic says Audi's are the highest maintenance as well.
The best one of these cars for a young male is definitely the BMW or Audi. Personally I prefer BMW, but Audi is also a fun, young person car. Out of the 3 Mercedes would be the least sporty and least preferable for a young driver.