K.T.
C.,
My son was a biter for a short period of time around 2 1/2, after another child had bitten him on the face. I made my son apologize to the child, the teacher, and the child's parent for each person he bit as well as get a lengthy talking-to about the inappropriateness of biting. This became tedious for him to go through all those steps so he stopped.
We also had a child (well before the age of 3) in my son's preschool who was a chronic biter. His parents gave him something like a teething blanket (soft square thing with rubber corners) and said "If you feel like to have to bite something or someone, bite on this". He was able to resolve his internal conflict by going to the blanket and biting it.
Since your son's doing this while you're not there and then you hear about it, have whomever is watching him at that time instantly act. 3 is plenty old enough to learn that biting is not acceptable behavior. Make a big fuss over comforting the victim while he's watching to see that his behavior caused pain and you're helping the victim feel better. Then take your son away and firmly tell him that biting is not the way to solve problems. Ask him if he can think of other ways to manage his feelings - get him involved in coming up with a solution.