Every triptych I've done has been seperate sheets mounted on a common board. That said I would approach your problem by trying to make use of the good old "speed-ez-el". An 8X10 speed-ez-el is open at both ends of the 8" section. So if you had 8" roll paper cut to 24" (8X25 with 3 8X8 frames) you could mask the 10" section to 8", shove the first section in, print, shove it through so the mid section is in, print that part, shove it through so the final section is in, and print with appropriate masking of the exposed paper that is beyond either end of the frame. Speed-ez-el's came in 5X7, 8X10, 11X14, and 16X20 sizes with the smaller end open at each end so you can slide paper in one end and out the other.

This AM I worked on a triptych of 3 4X5's that make up a common vertical scene. A small waterful in Yosemite. Contact printed together in a common frame on a sheet of 5.5X14" paper. It's a do-over as I ran out of time but it shows promise. So in that case it makes a roughly 5X12" vertical picture of the 3 frames. Kind of unusual.