Hi Pilou,

re:
> Say I have a selection of 3 volumes A, B, C
>
> does it yet existing something who automatically save them like
>
> objectA.obj, objectB.obj, objectC.obj ? or objectA.3dm, objectB.3dm, objectC.3dm...

Try the script from here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=8230.5

Note that you need to modify the script before you run it to specify the directory where you want the files to be written to.

- Michael