do you mean a triple alpha?
if so the dimensions are 3384x2175 with a 314 pixel spine. With a little simple math this wouldn't be that hard to create in Photoshop. Just go to View->New Guide and write in the at what pixel you want the guide and end it with "px"
ex. "1692 px"
didn't know people still used alphas.
My covers may only be viewed on well calibrated screens and printed on well calibrated printers.