I have not played GTA3, but I have a very good idea of how video games work in general. Yes, people who are not major gamers often comment about how good GTA3 is because of the violent comment. But the thing is, a would be willing to bet the violence is not why they enjoyed the game, but the fact that the game itself was very good. It had freedom to do what you wanted, presented challenging mission based goals, and had a good engine.
People who don't analyse games don't realise how these things interact to make a good game, and so cite the violence as the reason they enjoyed it. But there's more to it than that. Case in point: Look at all the bad games out there that are violent, but don't receive so much attention.
People need to take personal responsibility. You played a game, so you shoot people? Anyone who doesn't have a severe mental defect can tell the difference between real life and a game, and if someone was mentally ill, anything, be it a game or movie, could have set them off.
Finally, Video games are not toys. It's time parents started taking some responsibilty, such as not giving five year olds games marked "15+" Would the same parents let the child watch a horror or pornographic film? Hopefully not, although now I think about it, some parents are that irresponsible.