I believe you meant Galaxy S3? Lol...There are pros and cons to both models. Firstly, if you are talking about the iPhone 4, the galaxy S3 wins hands down as it is much faster and can even go on the 4G LTE network, on top of having a bigger screen and much better camera. The iPhone 4 came out approximately 3 years ago whereas the galaxy only recently just came out about 2 months ago. Secondly, if it's user experience you are after, I suggest that you get the iPhone, as not only does it have much more apps and can do things better than Samsung, it's not as cluttered and easier to use as well.
by the way an update, I just checked online, and unless I misread, there's no such Galaxy model called the 'Y3', it's only the Galaxy Y or Galaxy S3