I take it back. The OT3-iest threesome that ever 3-ed isn't Samurai Champloo or Hancock. It's Singin' In The Rain. Seriously. Go look up
the Good Morning song and tell me that's not a scene that fairly screams threesome. The scenes immediately preceding and following only serve as further evidence: It takes both Cosmo and Kathy to console Don over The Dueling Cavalier! They stay up the whole night talking! Cosmo comes up with a way to save Don's career and boost Kathy's! Cosmo and Kathy snuggle on the couch and Don doesn't care! Kathy makes out with Don and Cosmo! (Seriously, not making this up.) And then every scene after that there is no Don-and-Kathy ever, it's always Don-and-Kathy-and-Cosmo. They're totally in love, guys.
Also, it doesn't help that, apparently,
95% of fic for this movie are either threesome or Don/Cosmo. Sometimes I really love fandom.