Civ3 units are precompiled animation files. The base mesh can not be extracted. So, the only way to get a civ3 file in to civ4 is to talk the modeler (which might or might not be the unit creator) to make a low poly version and share it.
With rare exception, Civ3 units are very high poly and very high texture details and so just talking the modeler out of the mesh would not be enough.