The old truism that actors always die in threes sadly proved itself again this weekend. After last week's passing of Italian leading man Vittorio Gassman, 77, came this weekend's deaths of two longtime Hollywood favorites, Walter Matthau, 79, of a heart attack early Saturday morning in Southern California, and Harold Nicholas, also 79, of heart failure following surgery in New York on Monday. Matthau shot to stardom in the mid-'60s when he reprised his Broadway role as the sloppy Oscar Madison in the screen version of "The Odd Couple." He was last seen playing (what else?) a lovable old crank in this year's otherwise forgettable "Hanging Up." As for Nicholas, he and older brother Fayard (who survives him) formed the Nicholas Brothers, probably the greatest tap dancing duo the cinema has ever seen. Their star reigned in the '30s and '40s. Off-screen, one of Nicholas's three wives was the tragic superstar Dorothy Dandridge.