Three Universal Frankenstein films featured Boris Karloff as Frankenstein’s monster: Frankenstein, Bride of Frankenstein, and Son of Frankenstein.
Karloff first played the creature in Frankenstein, released in 1931 and directed by James Whale. His performance, combined with Jack Pierce’s distinctive makeup, created the screen image most audiences associate with Frankenstein’s monster: a flat-topped head, neck electrodes, heavy clothing, and a stiff, lumbering walk.
Karloff returned in Whale’s Bride of Frankenstein in 1935, then appeared in Son of Frankenstein in 1939. These are the three Universal films in which he portrayed the monster. He did not continue in the role through the later cycle of Universal monster films.
The usual mistake is counting every classic Universal Frankenstein film rather than counting only those with Karloff as the creature. Lon Chaney Jr. played the monster in The Ghost of Frankenstein, Bela Lugosi took the role in Frankenstein Meets the Wolf Man, and Glenn Strange played it in several later productions. Karloff later returned to the Frankenstein series as the scientist Dr. Gustav Niemann in House of Frankenstein, not as the monster.