The title "House of..." could refer to the ruins/house owned by Ludwig Frankenstein, the second son of Henry Frankenstein (portrayed by Cedric Hardwicke) in The Ghost of Frankenstein (1942). It's also the same "house" where Lawrence Talbot discovers the Monster in ice in Frankenstein Meets the Wolf Man (1943); and, of course, where Neiman discovers the Wolfman and the Monster in this film. (The castle is entirely washed away in the flood at the climax of " - Meets the Wolf Man," but is inexplicably semi-intact here.
Glenn Strange was the fourth actor to play the Monster in Universal's Frankenstein series. The actor who played the original Monster, Boris Karloff, was also present in the film, playing the role of Dr. Niemann. Being on the set, Karloff was able to personally coach Strange in the way the Monster should be played.
Bela Lugosi was slated for the role of Dracula, but the film was dependent upon the presence of Boris Karloff being released from the stage tour of Arsenic and Old Lace (1944). Shooting was delayed, and John Carradine was cast instead of Lugosi, who had a prior engagement: ironically, playing Karloff's "Jonathan Brewster" role in another touring company of "Arsenic and Old Lace."
This is the first of the Universal Frankenstein films in which a member of the Frankenstein family does not appear, unless you count the monster himself as part of the family. According to The Bride of Frankenstein (1935), his name is Frankenstein as well.
Of the 5 'Wolf Man' films featuring Lon Chaney Jr.; this is the only entry that does not feature the Wolf Man's growl or howl. It may be there but, if it is, it's drowned out by Hans J. Salter's music score.
The archetype of the Hunchback and his unrequited love for the kindly Gypsy dancer comes from the classic French epic novel Notre Dame de Paris (often called The Hunchback of Notre Dame) by Victor Hugo.
Although this film boasts three monsters, the Dracula scenes are completely separate from the Frankenstein and Wolf Man scenes. A true meeting of all three monsters would have to wait until House of Dracula (1945).