For a Windy City attorney, the clues to a triple murder lie somewhere between the sinking of the Titanic and the dead letter office.   Rodney Fairfaxx is influential and wealthy enough for his family to ignore the Chicago magnate's one little quirk: Ever since his sweetheart sank with the Titanic three decades ago, he's been anxiously awaiting her letter assuring him she's dry, on the mend, and coming home soon. Now, attorney John J. Malone is defending the harmless eccentric against a charge of murder. The victims: three postmen, clubbed to death. The alleged motive: failure to deliver.   Malone's good friend, socialite Helene Justus, has been close to the Fairfaxx family for years and she's not buying it. However, her intuition may be as unreliable as the mail. The respectable clan yields a cluster of certified suspects--from Rodney's crafty niece and nephew to the tight-lipped housekeeper to Rodney himself, who may not be as off the beam as everyone thinks. But as tensions rise and secrets are revealed, another murder sends Malone on an entirely different route.   With The Fourth Postman, author Craig Rice delivers "plot and people as wacky as ever, with . . . plenty of comedy and, surprisingly, much intriguing sleuthing. Verdict: Fun" (The Saturday Review of Literature).   The Fourth Postman is the 5th book in the John J. Malone Mysteries, but you may enjoy reading the series in any order.