One thing I've learned about 3rd graders....they think they are right. They don't mean to argue with adults, it's just...you are wrong and they are right and they are just explaining. They are doing you a favor. You should be grateful. This is their perception, and you know what they say about perception...it is reality.
So I've been training myself to rephrase things in a way that stops any arguing, excuse me, any student lead explanations of misbehavior. I found a great guide that has helped me rephrase my words, it's from Love and Logic of course!