I think I have done what you are trying to do, but I am not sure. I have top level objects listed across the top of my site (in tabs). When you click on one of the tabs, the pages within that section appear in a left hand nav list with the section name at the top. If this is what you are trying to do I can post some code...
It may be... I have top nav on top, and left hand sub nav working fine - but the left hand subnavigation doesnt expand out to show the third level navigation, and this is the bit I want to add.
If you have code that does this that would be marvellous!
Try this...I have the depth set to 4 (four levels deep) but that is adjustable. In fact, that might be the only thing that I changed. This file is called flat_left.tpl and is located in mydesign/override/templates/menu folder.