Most Ragtime pieces have 4 or 2 beats in a bar, and some are even called "Two Steps". Scott Joplin and other ragtime composers have experimented with adapting ragtime principles to 3 beats in a bar, which in a sense anticipates the concept of the "Jazz Waltz". Bethena (subtitled A Concert Waltz), is a perfect example of this and was featured recently on the soundtrack to The Curious Case of Benjamin Button to herald the end-titles, and capture the final mood of the film.
Here is a video of Bethena. Scroll down to see and play the Sheet Music.