The only reason you should really wait is because of the new gta game coming in October (or so I've heard). The only thing GTA3 is really lacking is a large in game map like Driver 2 and multiplayer. Other than that, it's perfect in every way you could think of. If you don't want to wait months for the next one, go ahead and get GTA3 and play it to death, then get the new one when it comes out.
It's really a great game with tons of replay value, and there's really no reason to not get it, unless you have a very weak stomach and can't take the way the controller vibrates when you run over somebody. Also, it's DEFINATELY NOT A CHILD'S GAME. No question, not for children.