All of the fiberglass floorboards I have seen are made for four speeds. They are made, however, so you can use a small piece of material to close the hole down to the size you need for a three speed.
Recently, one of the members on this site, A. Chipmunk, lead me to a site of a company which makes steel floorboards. I don't know how close they resemble the original or what the situation might be with the transmission hole or the battery access hole, but it might be worth looking into. They are:
Direct Sheet Metal and their web site sddress is:
http://www.directsheetmetal.net I also know that battery access hole covers are being reproduced, so a person could cut a battery access hole into their floor board and use the reproduction battery access cover if they don't come with access hole in them. The 1/2 ton truck floorboards also have an access hole for the master cylinder reservoir. The larger trucks do not because the master cylinder is located closer to the frame rail on a larger truck. The floorboard in my truck came out of a larger truck and I cannot access my master cylinder unless I take up the floor board.
That brings up another point, if you decide to look for an original floor board and/or a toe board, don't rule out the larger truck cabs because all of the cabs were the same in size. And don't forget, 37, 38 and 39 are all the same. Cars and trucks are not, however, the same.
If you decide to contact Direct Sheet Metal, let me know what they have and if they can give you a picture. I would like to know for the future.
Ken, I wish you luck and if I can answer any questions or help you with your truck, please let me know.
Joe