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
Chapter 1-23. The Schlegel-Wilcox opposition is set up, and then cemented when Margaret decides to marry Henry; Helen comes out and says that she is breaking off from them.
Act II
Chapter 24-41. This act ends with the convergence of all of the bad things that have happened so far – Helen's pregnancy, Charles's Wilcoxian sense of propriety, and poor Leonard's final appearance. Of course, it all ends up at Howards End.
Act III
Chapter 42-44. After Charles inadvertently kills Leonard, the last couple of chapters tie up all of our loose ends – and present an opening into the future.