You should shoot for 4.0 unweighted. Why would you shoot for anything less? Just do your best and "what will be, will be." If you aim high and fall a little short, maybe you will achieve your Plan B or Plan C. No one can give you an exact GPA, because the the type of classes has a lot to do with it. SAT's are perhaps a more objective measure. The academies all publish average SAT scores.
NJROTC gave the perfect answer. SPECIFICALLY the first 2 sentences. "You should shoot for 4.0 unweighted. Why would you shoot for anything less? ".
The academies are NOT just another college. They ARE the MILITARY. You aren't just going to school for 4 years. You're making a commitment of those 4 years and another 5 years after graduating to serve your country. Serving your country, especially as a commissioned officer, means being a leader and having other people's lives in your hands.
The academy and military are not interested in people who try to do "JUST ENOUGH" to get by or to be competitive. They want and expect you to give 100% in ALL you do. Does that mean you'll always succeed and be the best? No, not at all. It simply means, that at the end of the day, whether you get accepted to the academy or not; whether you achieve a certain goal or not; whether you win or lose; etc..... that you can hold your head up and KNOW DEEP DOWN INSIDE, that you gave it your all. That you gave 100%. That there wasn't one more thing that you could have possibly done to improve. If you can say that at the end..... then it doesn't matter if you receive an appointment or not. You'll be able to move on and succeed at whatever you do.
Sorry if I seem a bit passionate (Or anal) about this subject. But as an ALO, and having spent over 20 years in the air force, I can't tell you how many times I've been asked questions like this. What's the minimum GPA to get accepted? What's the minimum SAT/ACT to be accepted. What's the minimum amount of sports.... level of classes..... etc...
As I said before, the academy and the military don't want someone who's trying to "Get By". They want 100% effort in ALL THINGS. That's how you should be handling your next 3 high school years. Your goals are to have a 4.0 gpa unweighted in ALL CLASSES. 800 on your SAT tests. A 36 ACT score on each area of the test. PERFECT attendance. Be involved in as many school/social clubs, athletic teams, community activities, etc. that you can possibly schedule time for. (While still pushing for the 4.0gpa). These ARE YOUR GOALS. This should also be your GOAL FOR LIFE. When you get a job; doing your best. Not just enough to keep your job and get a paycheck. Raising a family. etc. BAD HABITS become a way of life. So do good habits. And before you think what I suggest/recommend is impossible to do all in a day....... THOUSANDS of academy applicants find a way to balance all of this every year. Applicants balance a FULL semester load of classes. Classes in the IB program, AP Classes, Honors, etc. They maintain 3.9-4.0 gpa's. They play 2-3 sports a year. They're involved in 2-3 school clubs and community activities. They find leadership positions in these activities. They still find time to spend with their family and social life with their friends. It definitely can be done.
Best of luck