In the story "A Rose For Emily" by William Faulkner, how can it be shown with three supporting points that Emily's attempt at controlling time is ultimately futile?

If it is your intention to show that Miss Emily, in "A Rose for Emily" (Faulkner), has been futile in her attempts to hold back time, that is, in fact, your thesis.  A thesis is simply the main idea you wish to prove.  A thesis statement, on the other hand, is a statement that states your main idea and your supporting points.  So it is the three points we need to work on here, and really, there are so many points, it is almost difficult to choose. Let's go over four that come to my mind immediately.
First, Miss Emily remains in her crumbling mansion, a household she clearly cannot afford to keep up, even not paying taxes, and it crumbles about her, the only house left in the "good" neighborhood when she dies, in "coquettish decay" (Faulkner 1), much like Miss Emily. She cannot stop the clock on this decay.
Second, Miss Emily, while she may perceive herself to be a winsome Southern belle, is seen by the townspeople as a "fallen monument" (1), obese, unattractive, with a mixture of black and grey hair. She has not held time still as she grows older, nor has she even aged gracefully or with dignity.
Third, I would not call her murder of Homer Barron a great success in holding off time.  One's imagination can only enhance a crumbling corpse for so long and then I'm sure a serious "ick" factor must come into play. 
Finally, Miss Emily herself dies, as no one can hold off time enough to avoid death.  In spite of her early pampered life and how she sheltered herself after her father died, every crumbling monument must finally crumble away.
To write a thesis statement, you need to select the points you wish to develop and support and then add them to the thesis itself. For example, if I were writing a literary analysis about this story, I could have a thesis statement like this:

Miss Emily's father ruined her for real life, by forbidding her any suitors, by not providing her with an education, and by not teaching her how to do anything that could have gained her employment. 

That is a different thesis, of course, but notice that I state it and then just add my three supporting points.  You have a good thesis to write about, and all you have to do is add the points you want to make to the thesis. Place the thesis statement at the end of your introduction, where the reader can now use it like a table of contents for the rest of the essay. Good luck!