I don't have a 3 year old, but I am hearing your anguish. It sounds like you're struggling with your own expectations too.

I think the only thing that kids really understand is who is in control. If you're yelling etc, then you're clearly not in control. That idea of 123 Magic sounds like it plays on the idea of calmness, and maintaining composure...NOT easy when you have a serious public tantrum or one child is hurting another.

I am at the start of all this toddler assertiveness, so i feel a bit useless...but just know you're not alone and you're doing the best job you can. Have a bubble bath or something tonight to unwind...