In August, I wouldn't go to Patagonia. Best skip it altogether: it's wonderful, but that is just the wrong season. As for Perito Moreno: that's a major tourist trap. It's just a large glacier; nothing else to see and do, and you get a better view watching Discovery.
Go to the northwest instead. It's culturally more interesting, less touristy in the bad sense of the term, the weather will be much more agreeable, nature is absolutely stunning, and it's cheap. Bearing in mind that you have just three weeks, a nice itinerary would be:
BsAs - Cordoba - Mendoza - San Juan - La Rioja - Catamarca - Tucuman - Salta - Iguazu - BsAs
The journey between Salta and Iguazu kind of sucks; however much I'm opposed to it, it may be quickest and easiest to take a plane (which will take you via BsAs).