Hi,
Unless those websites offer an Android version of their app, you will not be able to use an iPhone app on your Samsung Galaxy III. The Android app market is growing and getting to a point that most major app creators are making apps for both the iPhone and Android.
On the other hand, for browsing the web, if a website has been optimized for an iPhone, this really just means it's been optimized for viewing on mobile devices of a certain size. This means that "iPhone optimized websites" will most likely be optimized for your Samsung Galaxy as well.
To summarize:
- Apps: No (unless there is an Android app)
- Websites: Yes


