I'd be inclined to do three separate prints and mount them as a triptych.

You can handle the registration issue with a 4 blade easel, as Ian describes, or with a single 3-window mask, but what about dealing with the dodging, burning, and other manipulation issues for each frame? What if two look good, but you feel you need to work on one? If you print them on the same sheet, then you've got to do all three for every proof. Of course you could proof them separately and just do the final on the single sheet, but if you are going to use a 3-window mat, then it just seems easier to do three prints.