My understanding of the Three Act Structure is the following:
- Act I: The Status Quo is shown. For some reason the protagonist realizes that the status quo is not stable anymore and needs revision. Note that it is crucial to define the "initial conditions" of a story. Only then will the reader be able to fully grasp the importance and the significance of the change that you describe in Acts II and III.
- Act II: A change occurs. It might be wrought by the protagonist or by some other incident. The scene in which the change actually occurs is commonly called The Crisis. Note that the word crisis originates from Greek and can be translated into "separation", "judgement", "dispute" (see Wiktionary). That is: The crisis brings about a substantial change that will impact the unstable Status Quo identified in Act I in one way or the other.
- Act III: The change induced by the crisis is incorporated into the daily life of the protagonist. Act III thus answers the important question: Will the protagonist be able to consolidate the change? Or was the change of Act II just a fling that fades away in the first light of dawn? The scene that usually answers this question is The Climax. (climax, Latin derived from Greek, literally "ladder", more generally the culmination of an evolution, see Wiktionary.) Note that, in my opinion, it is crucial that the protagonist is active in Act III. He may be passive in Act I, he can be passive in Act II, but Act III is the time for him to show what he's made of.
Of course, this structure is very general and needs to be adapted to your specific needs. The crisis and the climax, for example, do not need to be widely separated from each other. What I find very helpful, though, is the notion that the Three Act Structure is the most general description of a change. And that is what any one story should deal with: A change.
My advise for you would thus be: Identify the change that you want to tell your story about. What is the one crucial transformation that fascinates you and that needs to be shown to an audience? Is it a physical change, a social one, or a psychological one? Is it necessary to separate these three changes from each other? Can you weave a story about this change that allows you to bring about the three climaxes - heroine defeats a physical threat, resolves her issue with society and consolidates a fundamental psychological evolution in her personality - in one scene or at least in scenes that are close to each other?
In general, when it comes to the Three Act Structure, I found Chris Vogler's "The Writer's Journey" very helpful. You need to get used to his rather mystic language and do a lot of interpretation for yourself, but if you do, it pays off. A recent great example of the Hero's Journey well used in entertaining fiction (no literary ambitions here) is Jonathan Stroud's "Lockwood & Co: The Screaming Staircase". The structure of this book and the way it creates suspense is simply marvelous. On the other hand, if you are familiar with the other works of Stroud, you will find that he uses the same structure over and over again. He does that very well, but since his stories usually lack a pronounced psychological component, they repeat themselves. Hence, to answer your question in a purely subjective way: For me, the psychological evolution of a character is what makes me read a book and creates suspense most efficiently. I would always try to focus on this change.