Why would you try reasoning with a 3 year old when they dont know the difference between right and wrong?
There's nothing wrong with getting a spank and being sent to the stairs. It'll teach the kid not to do it again. It happend to me and I turned out fine and have a fine relationship with my parents. We're not talking beating the shit out of the kid, just one spank so they know not to do something again.
It's perverted. Why the ass? You're gonna give the kid a spanking fetish.
As metal maniac said the kids not going to have much pain/any damage if you smack them on the ass in comparison to, smacking them/punch in the stomach or hitting them in the face.
And the child would more than likely develop a spanking fetish for lack of being spanked as a child, rather than being spanked as a child.