Terrible three's

Why is it that when it comes to age, two year olds get a bad rep? I have never had problems with my two year olds. My trial begins when they are three. At least when they are two they are still little enough that they don't know they are doing something wrong, or at least don't quite understand the consequences. Emma, it seems, turned into a monster about the time she was turning three. She is always scratching and pinching everyone. Today she scratched Tayvens face. (he's one of the little boys I tend.) His mom was pretty upset, which I don't blame her. I'm pretty upset about it too. The two older princesses have scars from their encounters with their little sister. I just have no idea how to stop her. She even does it to her dad and to me. Spanking, time outs, long talks, and threats have not solved the issue. If she keeps doing it, Tayven's mom will stop bringing him. The most frustrating thing is that most of the time she is a sweetheart. I don't know what could be bringing out this naughty side of her. Is it just her age? Whatever it is, she's not going to like the consequence that happens next time she does it. I'm going to scratch her back and I won't be nice about it. We'll see if that tames the little beast!