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I know my solution probably is not the best but it definitly worked with my 6 year old. He was 3 at the time biting everyone for everything. I had tried everything from time out to taking his toys nothing worked. Then i finally bit him back. After about 3 times of me biting him back it stopped. He had bitten the neighbors daughter, so i told him that biting was wrong and it hurts as he tried to bite me for correcting him i stopped him and bit him on his arm. Not hard enough to leave a mark but enough for him to realize that it hurts. After the third time of me doing it back to him within a week it stopped completely. Never happened again.
My son quite recently went through this. He is 3 as well. There is a little girl next door he plays with and when he would get frustrated, he would bite her. Funnily, it was only her, not any other children. First thing we did is to teach him other strategies for dealing with frustration, and we practice. So when I would see him beginning to get frustrated (she took his toy, or whatever) I would immediately step in and remind him to use his words. The trick was to catch him as the frustration was beginning, prior to it getting to that point. We taught him things like "I feel mad", and taught him it is ok to feel mad, just not ok to hurt. It took time, but it worked. The second thing we do is immediate consequences. I learned this through the book "Love and Logic" (fabulous read!) So as soon as he did something physical, no warning, no yelling, no threatening, just immediately picked him up and he went to his room. We stay calm and tell him "you hurt somebody, now you need to spend time in your room". The results have been amazing. I personally think they are not old enough to play unsupervised. We dont' yet let my son play with friends in his bedroom. Kids this age do not yet have full control of emotions, and things can escalate very quickly. Being able to observe them playing and seeing his start to get upset can really help.
It's important I think for children to know they can stick up for themselves. But we can teach them ways of doing that. Your son is probably frustrated with something...perhaps jealousy or anything really. Watch for it to come, and keep practicing words and alternative ways he can express that. It's not a quick fix, but it's worked for us. And I highly recommend the book "Love and Logic". It is a common sense approach to discipline and works so well.