According to director Michael Mann, when Dillinger escaped from that prison in real life, it wasn't three guards he tricked into imprisoning themselves like they showed in the movie -- it was 17. If our math is correct, that means that reality is almost six times as awesome as what we got. Dillinger himself wrote in a letter to his sister that it was actually eight deputies and 12 trusties (prisoners with special privileges), bringing the total number of people he duped with a piece of wood up to 20.
Even more impressive was the time he captured an armored car by disguising a hoagie as a bazooka.
Mann, however, decided to make the scene less realistic because it was already unbelievable enough with three hostages. Johnny Depp tricking a bunch of guards with some ridiculous ploy that ends up working seems like something out of the Pirates of the Caribbean franchise, not a biopic based on reality. There's just no way to put Depp on screen tricking more cops than the number of bullets they thought he had into locking themselves in a jail cell without it looking silly.