Importing 3ds file in ldraw (LeoCAD)
.3DS is a 3D image file format, a rendering of what you designed in LeoCAD.
All information necessary to recreate your build (i.e. what bricks to use) will be stripped from it. I suppose that theoretically this information can be recreated for small builds, but for medium-sized to large builds, this will simply be infeasible.
Hence I think it quite unlikely that it's possible to import .3ds back into LeoCAD.