My three year old pit bull has bit two people for no reason, do you think if I neuter him he will stop

Neutering him should have been done long ago. Yes neutering him will mellow him out SOME but it may be to late. What were the circumstances surrounding him biting these people?

Why is the dog so aggressive ? Was there a reason for the bite,

I think all bites happen for a reason. Now it may not be a GOOD reason from our point of view, but the dog usually has a motive.
What were the people doing, and what was the dog doing when the bites occurred? Any common ground between the two episodes? On or off your property? Dog possibly feeling cornered with no escape route, or trying to protect his family or property? Did he show some body language telling the people to "back off" that they either could not read or ignored? Does he growl or threaten in any particular situations otherwise? (I once heard someone describe a growl as "a bite that hasn't happened yet".)
Definitely get him neutered. This will not be a magic bullet, but neutered dogs are slower to become agitated and they calm down faster. You need to find a trainer or behaviorist familiar with pit bulls to work with you. They are not dogs that respond well to rough handling or discipline; they need someone who can read their faces and body language and help you find what's going on.

I have a Siberian Husky who is 3 years old and last year right after he turned 2 I decided to get him neutered(before I started dog obediance). Now being a husky Rex had always been extremely hyper. When we brought him to the vet even to get hid nails done he would growl. Everyone thought it was a stupid idea to get him neutered they said he was to old for the surgery to change anything. After he was neutered we brought him back to the vet 10 days later to get the stitches removed and when they brought him out of the room they praised him highly and gave him aa treat they were amazed at the big change in his attitude.