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.
As narrator Buddy Glass openly admits, "Seymour: an Introduction" is as far from a typical short story as you can get. It's more of a collection of anecdotes and thoughts than a structured narrative, and so we won't even try to break it down into structured plot stages. However, we can do these typical plot analyses for "Raise High the Roof Beam, Carpenters."
Act I
Buddy goes to Seymour's wedding, jumps into one of the guest cars, and ends up inviting everyone up to his apartment.
Act II
Buddy flips out at the Matron of Honor and retreat to the bathroom.
Act III
Buddy reads Seymour's diary and gets drunk; we find out that Seymour and Muriel have eloped; Buddy tells Muriel's deaf-mute great-uncle the story of Charlotte's stitches before we move steadily into the story's conclusion.