I guess I grew up with parents who did the 1-2-3magic type style of parenting ...... and from MY experience as the "naughty child" it was completely ineffective. I copped so many wooden spoons broken over me that it wasn't funny. If your going to teach a child consequences to actions it needs to be done immediately not 2 warnings then strike. Sorry for the reference but you don't say to a naughty puppy 1-2-3NO, there is the "naughty" behaviour then correction straight away.
FYI 1-2-3 Magic doesn't actually advocate hitting or smacking, it's idea is to not use hitting and smacking (which are essentially adult temper tantrums).

Also, you don't have to count to 3 every time. For example, if DS pushes DD I will say 'That's 3 - time out'. No warning, but an immediate consequence that he has to have a break from playing with whatever they were playing with/doing. He goes to his room and he may not be quiet, he may not sit still, he doesn't have to, it's just a time out from the situation.