While I too am a long-time viewer of this ITV series and a much-avowed fan, I must disagree a bit with the previous writer who said this is not one of the series' best. "Three Act Tragedy" really is one of the best films of the long-running Poirot series. Agatha Christie's story, filmed once before for American TV as "Murder in Three Acts" (the original title of the first American publication of the book) in sunny Acapulco with an oddly contemporary setting, is a classic of misdirection with one of those twist endings the author is never properly celebrated for.
The story presented here is an absolute marvel of authenticity, with only a few changes made (Mr. Satterthwaite is deleted and the murderer's modus operandi is changed a bit here, negating the book's most evasive clue, "am worried about M"). Otherwise, "Three Act Tragedy" is pure perfection.
Directed with a combination of astonishing period elegance and artfully ironic noir camera work by Ashley Pearce (who directed the well-done "Mrs. McGinty's Dead) and scripted with great eloquence by Nick Dear (who also scripted "Mrs. McGinty's Dead" as well as two of the series' best later entries, "The Hollow" and "Cards on the Table"), it's hard not to be impressed with this film.
The slowness of the film's first half that the previous writer refers to is present in the book as well. This is due to the fact that the investigations, such as they are, are not manned by Poirot but rather by amateurs – amateurs who get nowhere fast (or slow) and provide more red herrings than usual for a Christie story.
What makes the film of "Three Act Tragedy" exceptional, though, is a tremendous ensemble cast, expertly led by the magisterial Martin Shaw (Inspector George Gently, Adam Dalgliesh, Judge John Deed and a bunch of other British TV detectives), who gives an astonishing performance of the performance of a lifetime here. Many others, including Kimberley Nixon as Egg, Kate Ashfield as Miss Wills and Tom Wisdom as Oliver Manders, give wonderfully notable performances here as well.
Suffice it to say, David Suchet is exceptional as Poirot, offering a performance that is as impassioned and world-weary as his character is written in the book. "Three Act Tragedy" really does rank among the very best work for all concerned in this terrific series, which still has a little way to go before "Curtain."