I have many times experienced that mods installed through a gta3.img installer such as Imgtool does not import mods correctly, so your issue may just be one of those. Try using another installer or a modloader for your local server and it should work. I have quit installing mods for my local gta3.img, loading through a modloader is 101 times faster and more convenient.
Use either Zmodeler, Autodesk 3ds Max or Blender 3d for modeling. I strongly suggest Blender > >Zmodeler > 3ds Max if you aren't student as 3ds is extremely expensive. Blender 3d is quite easy to get started with and you're able to learn the essentials of designing models with it. Zmodeler should be an okay program too, way cheaper than 3ds max too. It's mostly used for vehicle modeling AFAIK.