sometimes cap object doesnt create the desired results as the modifier cant understand what the model is so it guesses, to do this by hand in 3ds, select your groups and make them editable meshes, then right click and create polygons, in the 3d viewport you will see the vertexes on your selected objects and you can click 3 in sequence to make polys, once youve finished making a face or a poly, then just double click the last vertex and it will make them, if you clicked them in the wrong order then right click edit faces, then on the right list of edit poly features find the 'flip' on the normals tab that should make the selected faces inside out.