One of the major features of AllWebMenus, included in the "Type" group property, is the ability to set a menu subgroup to appear as a sliding submenu.
Slide function can be applied to any vertical subgroup as long as its parent group is also a single-column vertical group.
This feature offers many options in both submenu appearance effects - just like in a drop-down menu - and the sliding submenu positioning.
Check the menu in this page, click the "Company" item to see the sliding submenu (2nd level) in action then click "Downloads" and the 3rd level will also slide below it.
More info on Sliding Menu Add-in
Example I Example II Example III
this AllWebMenus project file.