'Because I'm the parent' is a perfectly fine answer for a three year old. You certainly can't go into some deep philosophical discussion with them at that age. For what it's worth, neither of my children ever even asked why they couldn't hit but I could spank them. They generally just realize it's a punishment for something they have done wrong.
There are worse punishments than a simple spank, like making a child feel worthless.
I believe a spanking should be administered if the situation calls for it.
I also believe a hug should also be administered when the situation calls for it.
Its true what they say it hurts you more than it does them.
Tony