For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
For all three of the novel's plotlines, the first act runs from the early morning until around lunchtime. In the morning, the protagonists' days seem bright and full of promise, but that slowly changes as their days go on.
Act II
In all three plotlines, the second act runs from the early afternoon to the early evening. It's during this stretch of time that all three of the novel's protagonists face the biggest trials of their days: Laura Brown disappears to a hotel for a few hours, Virginia Woolf nearly runs away to London, and Clarissa Vaughan witnesses her best friend's suicide.
Act III
In keeping with the established pattern, the third act takes place at the same of day in all three of the novel's plotlines. As each of the novel's protagonists gets through her respective evening, she comes to terms with the despair, disappointment, and tragedy that have shaped her days.