In the 3-pane view, the folders are in the left pane, the documents in any given folder are in the upper-right pane, and the text of any given document is in the lower-right pane.
If a folder has both documents and sub-folders in it, is it possible to see those sub-folders in the upper-right pane along with the documents, i.e., just like a Finder window?
No, it’s not possible. This view is not uncommon in applications like ours. The directory pane in Three Pane view only shows groups. The Contents pane only shows files.
Try View > As Split with Widescreen turned off. The hierarchy with groups and documents together will appear in the top pane and the preview of the selected item in the bottom pane. Or, alternatively, turn Widescreen on and the hierarchy with groups and documents together will appear in the lefthand pane and the preview in the righthand pane.