Your purpose is to create an argument that one cause is more likely than others and to illustrate how that is the case. Part of this process will require you to show how other causes are less likely. The thesis of this paper should emphasize that one cause is most plausible, and that the others are still plausible, but might not be as strong with the argument following in that vein.
In order to accomplish this purpose, you must do the following things in your essay:
Identify the subject, show readers the many possible causes, but choose the three most logical to focus on.
Find one or two of the three reasons and develop them as the primary reason(s).
In the end, you must identify which cause is the most likely. This will be your thesis and overall argument.