The Photoshop way explained below. (From an online tutorial.)
I personally use Perfect Resize from OnOne Software,simple and quick.
Turn tiling on and fill in the size boxes. Apply and save to folder,done.
Here is one way using guides make sure snap to guides is currently active using menu view> snap to.
Lay down guide line using menu view> new guide (Shift+Ctrl+R)
In the Dialog use Percentage
Horizontal 0% and 100%
Vertical 0%, 33.333%, 66.666% and 100%
Use the rectangle selection tool set to normal pull out a selection 0%,0% to 33.333%,100% Ctrl+C Ctrl+V Copy paste
re-target the background drag out a center selection 0%,33.333% to 66.666%,100% then Ctrl+C Ctrl+V Copy paste
re-target the background drag out a right selection 0%,66.666% to 100%,100% then Ctrl+C Ctrl+V Copy paste
You should not have four layers
The full pano background layer and three layers above it each 1/3 of the pano
Dupe the document three time and in the dups delete all layers but for one of the 1/3 so you have layer 1, 2 and 3 in their own document. Ctrl+Click on the layer icon in the layer palette to select the layers pixels the use menu Image crop