You can't save to the .3ds format - at least easily. To be honest I don't know why someone would want to - its a closed and obsolete format that has many limitations.
You can load .3ds meshes - once its loaded you can do what you like with the various mesh commands and then save to .b3d or .X using code from the archives.
You can load .3ds meshes - once its loaded you can do what you like with the various mesh commands and then save to .b3d or .X using code from the archives.