As I say, On the Waterfront was the first and The Godfather Part II was the last to go for three-for-five in Best Supporting Actor. But it did happen once before in the Best Leading Actor category. In 1935, all three male leads for Best Picture winner Mutiny on the Bounty were nominated: Clark Gable, Charles Laughton, and Franchot Tone. The next year, separate categories for supporting players were introduced. Oh, and in 1963, Best Picture winner Tom Jones got three supporting actress nominations: Diane Cilento, Dame Edith Evans, and Joyce Redman.
This much we can take to the bank: three acting nods in one category means your movie's winning Best Picture. But it also means one of the other two nominees is going home with the Oscar. Only once has a nominee from the threesome won, and that was Robert De Niro speaking Italian while "doing" Marlon Brando. In 1954, the honor went to Edmond O'Brien of The Barefoot Contessa - the only nominee from a film not up for Best Picture:
The nominees are: