I had to import Unreal .3d model to Blender and the following approach worked flawlessly for me:
Conversion pipeline (_a.3d/_d.3d) -> (*.obj) -> (Blender)
You will need U3Dbinread.exe that converts *.3d files to *.obj. Then you are good to go and import the *.obj into Blender using the native OBJ importer.
Here is the windows executable doing the heavy lifting: U3DBinRead
Note
In case someone finds the time to write a plugin importer, detailed specs can be found here.
The vertex mesh file format is described here:
http://web.archive.org/web/19991104160030/http://www.nervedamage.com/unfiles.htm…
The code to read vertex position is described in this file:
http://utgl.unrealadmin.org/UT_Package_File_Format_pdf.zip
on page 21.