1. "It depends on..."
Ex. Q: "Can you go to a movie with me on Friday?" A: "I don't know, it depends on when the movie is. I have to work on Friday also."
When someone uses "it depends on..." they are usually saying that in order for a request to be possible or answered, they first need to gather more information.
2. "It's depending on"
This is incorrect. I have never heard anyone say this before. However, someone might say, "It is dependent on...." which again means that the task to be completed or confirmed is reliant on something else happening or becoming known.
3. Depend on
To depend on someone means that you are relying on them for some reason. For example, babies depend on their parents because babies cannot take care of themselves. To depend on something means that in order to continue you must determine an outcome/ gain information about something.
More context would be helpful here in determining what these phrases mean. Also, please note that the above fragments are NOT sentences! These are phrases which can be used in sentances, but they lack completion. A complete sentence requires a verb and expresses a complete idea. Complete sentences will always make sense standing alone, whereas the fragments you have numbered above to do not make sense alone.
Hope that helps, feel free to ask if you have any more questions.