In AH2, it would be the terrain called "tunisia" (if you are selecting it in a custom arena), and the filename is "tunisia.res".
In AH3, it will likely be called the same thing, but the conversion isn't done yet for AH3. If you want in AH3 a terrain that is similar and has the ground installations we would have, you can load "italy" in AH3 and fly around from the part of Tunisia that is in that terrain.
