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.
Sorry, this play doesn't follow a three-act structure. Why, you ask? Because it's only has one act.
Act I
In three act structure, we know the first act is over when the characters finally commit to whatever we're going to be watching them try to do: the warrior princess swears to kill the three-headed monster; the lady decides this is the year she'll finally win the butter carving contest, etc.
Trifles is a short play, though, so our heroines don't totally do this until the very end when, after a lot of investigating and a little hemming and hawing, they go all in and decide to take the evidence that could convict Mrs. Wright. Sure, they haven't been totally forthcoming with it for a couple pages before the end, but they aren't truly committed until Mrs. Hale shoves that dead bird in her coat.