I'm not sure of the meanings by the expressions "bonnet d'ane" or "au coin". By "bonnet d'ane" do you mean "stubborn ass"? By "au coin" do you mean "on the verge of "," marginal case" ? I can think of a chinese expression though, which means "a stubborn stone nodding its head, which means a person being able to make a stubborn person understand, a hard case. I'm not sure of the meaning or saying you're looking for, I'm afraid this may not help much.
I've found many articles about pas 0 de conduite pour 3 ans. I'll describe the behaviour of a 3 yr old as " uncontrollable ", which I'm so familiar of.
Could it be "attention deficit hyperactive disorder" you're looking for, which is a common term/ diagnosis used to describe overactive young children?