Hi J.,
I might have more questions than suggestions, but here goes:
Is he in any kind of regular care or playgroup, or are his social situations more occassional? Some of his "problems" (like biting) are possibly more age-related. Is he very verbal?
I think positive praise is one of the best ways to reinforce good behavior. Tangible rewards might be a problem in the long run if he starts to expect "something" anytime he does behave the way you want him to.
Sharing can be difficult for preschoolers. My son will also be three soon and he does better if he initiates anything like sharing (with encouragement). If I make him "share," it is pretty rough. Sometimes encouraging something that does not involve toys can help with group play, like physical activity. There is nothing to share and the kids enjoy the adtivity together. If you are getting together with other children, you can try listening to music that tells the children to do things (like the hokey pokey, freeze dancing, jumping, marching, running in circles, touching toes, etc.) or follow-the-leader type games where you take turns copying each other's action. This might be a way to ease him into social activities without worrying about actual contact or toys. Keep trying but don't push him. I never liked kids when I was little and preferred to play on my own--not all children are outgoing and sociable, and that is okay. :)