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
Patching together a linear plot from Tristram Shandy requires some Memento-style maneuvering. If the point of Tristram Shandy is for the narrator to tell the story of his life, then Act 1 would take us up through about the middle of Book 6, at which point Tristram's wounded penis has finally been bandaged and his father has resolved to hire him a tutor. Now the heir to the family, the way seems clear for him to become the hero of his own story.
Act II
Tristram shifts gears near the end of Book 6 to turn his attention to Toby. Next thing you know, he's narrating his Grand Tour through Europe.
Act III
Toby's love affair seems like it's going to bring the story to an end, because Tristram has been promising to tell us the story since Book 1. Of course, the love affair fizzles out, and we never get a resolution at all.