The Uneasy Three

A crook, his moll, and his moll's dimwitted brother set about stealing a valuable jewel during a high-society party. They impersonate a trio of musicians hired to play for the affair, and when the real musicians show up, the gang must not only convince the party-goers that they can play the instruments, they must also persuade the party-goers that the real musicians are crooks out to steal the jewel. (imdb)