I have 3 sons (now 20, 15 and 12) and have never been a spanker; my personal feeling has always been that hitting people is unacceptable, and that for a child to be hit by the very person whom they love and depend upon above everyone else in the world amounts to a betrayal of trust. It also sends the dangerous message that it's okay to use violence as a tool if you're bigger/stronger/more powerful, etc. than the other person; too many people make the mistake of confusing "respect" with "fear"...they are *not* the same thing, and what thinking parent would want their young child to be afraid of them? Finally, it's been shown time and time again in a number of studies that spanking does nothing to deter bad behaviour, but is actually more likely to promote aggression at worst, and have little if any effect at best.
NOW---having said all that, my gut feeling is that this proposed law sounds a bit weird: for starters, why the "under age 3" business? Are we to infer that it's okay to start beating on Little Johnny as soon as he turns 4? Additionally, making something illegal by no means guarantees its extinction...lots of things are illegal, and yet people still drive drunk, smoke pot, jaywalk, etc.; child abuse is already against the law, isn't it?
Introspective and sensitive parents know that they shouldn't be hitting their children and generally behave accordingly. Meanwhile, those who think it's okay to spank will continue to do so, just not in front of you in the supermarket checkout! Are all parents who use spanking as discipline abusers? Certainly not. But the danger inherent in this type of legislation is that the people who DO abuse their children are already in violation of the law; another statute on the books isn't going to make the difference. If the aim is to make people think twice before swatting their kids' behinds in the mall parking lot, then it may well have its desired effect; how much change will occur behind the closed doors of private homes is open to question.