The SIII does appear to have a customised Samsung app along with the calendar (which I personally don't like).

I've not found a way to install the stock Android calendar but then I haven't looked all that far. It's not on Google Play that I can see.

If you disable the Samsung app you'll probably lose the instant contact call. Can't remember what they call it. The feature that lets you have a contact open and just put the phone to your ear to call.