I have a three year old and he loves to make friends and this kid next door is double his... he is constantly teasing him and hitting him when he thinks I'm not watching. (my son is not the type to complain unless something 's really hurt him bad so I have to keep an eye out on him if other kids are being unfair like this one) when my son hits back he complains by crying out his lungs and then his mother is all over him with love and sympathy then its her complaining that I have to face.
She is a nice person but this stuff is getting to my head I have no idea how to deal with this? Advice please!?

When you say "double his" are you saying this other child is 6?

When you see it, tell him he has to go home as hitting and teasing are not allowed at your house. If he complains, tells mother, and she complains, explain the situation to her. Calmly, matter of fact, but do not allow your child to be subjected to the bullying of an older child. Let this other child's mother know what is going on. If she doesn't follow-up with her son, then you may have to find another playmate for your son or be extra vigilant when they are together.

Invest in a video camera. In a situation like this it can become your friend. When little soandso says he didn't do it after you said he had you have proof to show the other mom. Now in the end she may hate you for pointing out flaws of her perfect child but it will also teach your child a valuable lesson on telling the truth.

Yes video it and tell them kids are being kids and parents at time do better staying out of it

You need to explain to the other boy's mother what is happening. She needs to know what her son is doing because it is her responisiblity as a parent to teach her child how to behave. If the child continues to bother your 3 year old, don't let them play together.

I think more supervision of these two playing together would be the best way to go for now. Either her, or you, so this can be nipped in the bud before it happens because the problem isn't the kids, it's the parents. You know what's going on because you are watching but by her only reacting after the fact she can only assume what has happened. Instead of getting into a big thing with the parent, limit there play together, and make sure they both know they are being watched. This puts them both on best behavior. Especially when one is bigger or older. Especially when parents can't get on the same page. To me that's the bigger issue to be solved.