Are you saying that you have a single image, let's say in landscape orientation, that you want to divide into three sections and then print and wrap each section individually so you can hang them in a triptych?
If that's the case, then you need to figure out how big you want the final piece to be and up-res if necessary to a pixel resolution suitable for canvas. You can probably do this by slicing your image first, then up-resing each section individually, but that will be a pain if you decide you want to change the slicing. It may be more burdensome (computationally, if your machine does not have a lot of RAM and you want a big final image) to up-res the entire image, then slice a copy and keep the full up-res'ed one as a master image.
As far as prepping for a wrap - find out from the printer how much wrap is required (1 or 2 inches, maybe?) around each edge of the image. You can simply extend the Photoshop canvas (Image > Canvas Size) and fill the extended area with pixels from the image near that edge - some folks mirror the edge pixels, some smear them, whatever.
If you need a plug-in to help and you will be doing this kind of thing frequently, OnOne makes a photoshop plug-in called Perfect Resize - I think the newest version of it may have a different name, but same concept. Attached is a screenshot - see how the wrap is automatically created and, in this case, reflects the edge pixels onto the wrap areas. Automagically!
I hope I'm not misinterpreting what you want to do - the best bet is to find a good printer who will work with you to achieve your intended output. Their process may have specific requirements that you will need to follow anyway, so best to get them involved early in the process so you don't waste a whole lot of time and effort undoing mistakes you could have avoided.
good luck!
kirk

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