It depends on how many miles you do each year and whether you use fixed or variable service interval.
In my particular case with my last A3 I did around 14k miles every year and it was on variable service interval which came up at 18k miles. So in 3 years I would have done 42k miles, I would have only paid around £300 each for two services at 18k and 36k. The next one would have been due at 54k which would have been outside the 3 years. £21 per month for three years would be £756 but I only actually paid out £600. Service costs may have increased a bit since I had mine done in July 2013 but not by another £75 for each service so I was better off paying for the services as they were needed.