Why are Dracula, the Wolfman and Frankenstein’s monster often lumped together? When did this start?
Well, Universal Pictures released DRACULA and FRANKENSTEIN within nine months of each other, in February and November of 1931, respectively. The two have often been paired as theatrical re-releases, in network teevee licensing, and in video and DVD release.
(THE WOLF MAN came along 10 years later, also a Universal picture, in 1941.)
The Frankenstein Monster was played by Boris Karloff in FRANKENSTEIN, THE BRIDE OF FRANKENSTEIN, and THE SONE OF FRANKENSTEIN…the role was taken over by Lon Chaney Jr. in THE GHOST OF FRANKENSTEIN, and by Bela Lugosi in FRANKENSTEIN MEETS THE WOLF MAN.
Chaney was best known for playing Lawrence Talbot, the Wolf Man (he also starred in SON OF DRACULA); Lugosi for playing Dracula (and he played a “Wolf Man,” though not THE Wolf Man, in ISLAND OF LOST SOULS).
By the time THE HOUSE OF FRANKENSTEIN came along in 1944, Universal was shoving Dracula, the Wolf Man, and the Frankenstein Monster together in one picture.
…to answer your OP, blame the marketing department of Universal Pictures. Universal also made up the Wolf Man story out of whole cloth, and hundreds of years of European lycanthrope folklore…(and the screenplay was written by Curt Siodmak, best known for DONOVAN’S BRAIN, a somewhat Frankensteinian tale.)