Actors in the Most Best Picture Winners

Photo: All About Eve, both Farnum and Flowers appeared in this winner

These actors have appeared in the most Best Picture Oscar® winners, just those in 3 or more:

Frankie Farnum (6): The Life of Emile Zola (37), The Lost Weekend (45), Gentleman’s Agreement (47), All About Eve (50), The Greatest Show on Earth (52), Around the World in Eighty Days (56)

Bess Flowers (5): It Happened One Night (1934), You Can't Take It with You (1938), All About Eve (1950), The Greatest Show on Earth (1952), Around the World in Eighty Days (1956)
Note: Bess appeared in 21 nominated films, the most by far. Ward Bond is 2nd with 11.

Wallis Clark (5): It Happened One Night (1934), Mutiny on the Bounty (1935), The Great Ziegfeld (1936), You Can't Take It with You (1938), Gone with the Wind (1939)

Robert Karnes (4): The Best Years of Our Lives (1946), Gentleman's Agreement (1947), All the King's Men (1949), From Here to Eternity (1953)

Edwin Maxwell (4): All Quiet on the Western Front (1929/1930), Grand Hotel (1931/1932), The Great Ziegfeld (1936), You Can't Take It with You (1938)

All of these were in three each: Irving Bacon, Billy Bevan, Ward Bond, John Cazale, Eddy Chandler, Heinie Conklin, Gino Corrado, Donald Crisp, Harry Davenport, Billy Engle, Pat Flaherty, Morgan Freeman, Clark Gable, John Gielgud, Hugh Griffith, Jack Gwillim, Jack Hawkins, Bernard Hill, Dustin Hoffman, Eddie Kane, Diane Keaton, Shirley MacLaine, Jack Nicholson, Lee Phelps, Talia Shire, Joe Spinell, Meryl Streep, Ian Wolfe

Morgan Freeman was in Driving Miss Daisy, Unforgiven, and Million Dollar Baby
Dustin Hoffman was in Midnight Cowboy, Kramer vs Kramer, and Rain Man
Diane Keaton was in Godfather, Godfather II, and Annie Hall
Jack Nicholson was in One Flew Over the Cuckoo’s Nest, Terms of Endearment, and The Departed
Meryl Streep was in The Deer Hunter, Kramer vs. Kramer, and Out of Africa

Many other famous actors including Laurence Olivier, James Stewart, Peter O’Toole, Gene Hackman, Robert De Niro, and Ben Kingsley were in two each. See the link below.

From Wikipedia - Actors appearing in 2 or more Best Picture winners, 81 in all:
Actors in Multiple Best Picture Academy Award Winners