Hello,
To me, building galeons depends of the mission number and the nation. There are 12 different types of boats numbered 0 to 11. Galeon is 0.
Look for SNO files in the subfolders of \data\missions. Try to edit one of them and to find a parameter named BUILDABLE_BOATS.
For example, in file \data\missions\MIS01\RED\MIS01RED.SNO : BUILDABLE_BOATS= 2,3,4,5,7,8,9,11. In this scenario, Red nation in mission 01 will not be able to build galeons.
Now back to your problem, SNO file for the 3rd french mission is \data\missions\MIS03\BLU\MIS03BLU.SNO. In line 27, BUILDABLE_BOATS=1,2,4,5,7,8,10,11 so you should not be able to build a galeon. Only England (Red) and Spain (Yellow) can do it in mission 3.