after you make your changes to the .psd
just slice it into 3 sections. if the sizes become a bit different when you're done, just make some changes in the templates to the image sizes. if you don't slice, you can always crop the image 3 times to get the same desired effect.
if you're not sure where to make the image size changes, search the templates for the image name, and you will see the image sizes in there. keep in mind each image has 2 sets of numbers that will need to be changed.
good luck