Why does Stockfish website offer many apps to download? I want to get a strong engine to practice while I'm not home. Look at: http://stockfishchess.org/download, we see a list of apps to download for the Stockfish engine.

Why are there three apps to downloads for the same engine? Which one should I choose? What are their advantages and disadvantages? Which one is the strongest? Which one is the easiest to use? What functionality do they offer?

PS: I have an iPhone 5 and Android tablet.
On the Android tablet, you download the version for Android, and for the iPhone you can download the iPhone version. 

Stockfish engine's source code is written in C++, and therefore it has to be compiled for the platform (Windows, Android, iPhone, etc.) on which it will run. Thus, you have different "versions" (not in the strict sense) of the same engine.  

For Android, I recommend you download the app called DroidFish. I use it myself. It is very convenient.