You would need two only cut two darkslides like Robert is talking about. The first is cut out for the one third on the right as you face the back of the camera. You expose that third first. Then you have marked on the darkslide how far it has to be pulled to uncover the left third. now the slide is covering the first exposure and middle section of film. After you make the far left exposure you put in the dark slide with the center cut out and expose the middle section. Then the full darkslide goes back

Like Robert said, this way you could use different emulsions in each holder.

you could also make a small jig to cut the film in the dark with a razor knife. Just need to use an oversize straight edge to hold the film down and keep fingers away from the blade and a couple pieces of plasitc to form a corner to align the fim.