Pitbulls for some reason at the age of three get a bit more aggressive. I learned this from my vet because I own 4 pitbulls and have experienced your situation with only 2 of them. It can be scary but your dog will calm down soon enough.
Has your dog had any training? Have you talked to your vet? You may also want to go to your local pet store and buy a book on pitbulls. They are very informative and help you to understand your dog much better.
I hope though that you do not give your dog up. Most people do not want pitbulls.
Let me know how things work out.