Put the first level of the hierarchy in the leftmost list (which
should use single-selection semantics). When the user selects an
item in it, show that item's children in the next list to the right.
Do the same with the child items in this second list, ande show its
selected item's children in the third list.
Once the user reaches items with no children -- the "leaf" items,
as opposed to "branches" -- you might want to show the details of the
last-selected item at the far right. The Mac Finder shows a thumbnail
picture to represent an image, or the first few words of a text file;
you might instead offer a
UI for editing an item, or for reading its content, or whatever is
appropriate for your particular application.
A nice thing about this pattern is that you can easily associate
buttons with each list: delete the current item, move up, move
down, and so on. Many toolkits let you do this in tree controls
via direct manipulation, but for those that don't have treeviews,
this is a viable alternative.