BE CAREFUL! The specs you list for your tub, 92x92x39 is likely the length x width x height. When ordering a hot tub cover, you will need to specify the corner radius, assuming your tub has rounded corners. When you do so, be sure not to confuse the 39" dimension with the corner radius dimension or you will wind up with a cover that will not fit!
You should be able to order a three panel cover from anywhere. You should expect the price to be higher than a two panel cover, because 50% more raw material will be used to make the three panels.
You may be able to mitigate this, if a dealer will work with you, as the foam comes in 96" X 48" sheets. One of those COULD be used to make BOTH 23" panels.
Also, be thinking about how you want the three panels attached, if at all. I recommend attaching the 46 to the 23 with a standard cover hinge, and have the remaining 23" panel separate. Some manufacturers can make a three panel cover so it will fold up like the letter Z, but this may not work due to your overhead limits.
Hope this helps!