This is not going to answer your question but I've seen some very nice panoramic images which have been printed separately and then combined/mounted within a larger frame with an obvious gap between them. If you have a subtle seam it'll tend to draw attention whereas an obvious gap (it doesn't have to be large) will tend to make people move backwards to view the three prints as a whole. For large prints and when done well it's quite effective.
Failing that, a variation on what Donald has suggested is to make enlarged copy negatives from 35mm and contact print them. You may find this easier to align as you're dealing with the full size print instead of a 35mm negative.