I can't think of a single situation across the entire world that would bring me to believe that shooting a three year old is justified. Violence is rarely an effective solution to any situation. Nothing that could happen in a road rage scenario is worth a life, especially one so young.
I can't imagine for one second that a person's poor driving is an excuse to take a life. That's why we have a police force. Everyone has had an experience with drivers driving improperly. If we all got out and just started shooting at one another, we would all be dead.
No, there is no excuse for shooting a 3 year old, even if it is accidental. There is nothing that a young child could do, or even be stuck in the middle of, that would justify an adult acting so rashly that they hurt the child. A person should take a breath before they act in anger.
No matter the age of the victim, road rage is never justified. It is a completely avoidable occurrence, and often results in the death of children who are innocent bystanders. Regardless of the situation, the act of killing a 3-year-old over a traffic conflict is never justified. This growing trend needs to be stopped.