A lot of that is determined by the individual departments. E.g. engineering department; behavioral science department; etc... As Hornet mentioned previously, a lot also depends on the schools available for you to apply to.
AFIT; (Which is the Air Force's Graduate School) at Wright Patterson AFB, deals mainly with engineers. And while the 3.0GPA at the academy may be enough to consider you for that Grad school opportunity right out of the academy; you wouldn't be able to apply or qualify for say the Rhodes, Marshall, Truman, RAND, Hertz, etc... Their gpa standards are much higher.
Also, many of the grad school slots that you are "COMPETING" for, are not just against other cadets. For a grad school slot at MIT, you are competing against individuals nationally and even internationally. Do most of the schools reserve slots for certain students; e.g. Air Force Academy grads? Yes. But that's totally their prerogative. The RAND, where Hornet went, only takes 3 AF Academy grads per year. They take approximately 24-27 total students. And those students come from all over the country and world. But they do take at most, 3 air force academy grads each year. Can they change these numbers? I'm sure they can. They are a private corporation. Just like Harvard, Cambridge, Oxford, USC, etc,....
FWIW: If you become interested in applying to Grad school directly out of the academy, there are individuals in your department (Your Major) who will be able to help you and help determine if you'll qualify. If you do qualify to apply, you'll even have an adviser assigned to you to help you with the application process.