http://fueleconomy.gov/
A friend of mine uses another site called "fuelly."
Been logging my fuel usage for quite some time now. You can just browse to see what real-world drivers are getting in nearly any make & model. And you can register, save your gas receipts, and track your mileage. I've had some insane variances between fill-ups, but the goofy ones are the 2-3 gallon top-offs where your "average mpg" is bound to be skewed.
EPA estimates 18/25 for the '09 MS3. I have managed to average 24 overall, which includes spirited driving on a fairly regular basis and A/C on most of the time. (FL living requires A/C) My daily commute is about 70/30 highway/city. Also one road trip to NC in there.
If I discount fill-ups of less than 8 gallons, my worst tank was 20.7 and my best was 30.0 in pure highway driving. On my daily commute, the best I've averaged for a tank is 25.6. I did hit 18.2 on 2.3 gallons while driving the Dragon in NC, and 38.6 on 2.5 gallons on a pure highway leg returning from NC. (I only filled up because others in my caravan had forgotten to do so before we left & needed to stop after less than 100 miles)