You have to do some scripting to have taskbar3 going down. You have to add a line and compute the new size (height) of the taskbar each time a tile is add/remove.
Take a look at the code of my "Popuped" theme:
http://www.litestep.net/index.php?section=2&action=details&id=122
It doesn't use lsbox, but it can easily be adapt.