DXTBmp only handles 2D textures I`m afraid
3D textures are different internally and if you edit with DXTBmp I suspect the output will be a 2D texture that won`t work the way expected.

I assume that by 3D you mean "cube map" as this is a DDS option that is available... just not in DXTBmp. I haven`t yet had the need to look into how cube maps work.