Desired action: Dropdown list 1 contains choices such as Ford, Chevy, VW,
etc.
If user chooses Chevy, the cursor is automatically advanced to a second
dropdown list with choices such as Corvette, Malibu, Suburban. If user
chooses Malibu, the cursor is automatically advanced to a third dropdown list
with choices such as blue, red, green, etc.

If user executes an ALT-click (or ctrl-click, or shift-click, or whatever
while positioned on any menu item at any level of either of the three lists,
that selection item gets populated into the final, destination cell.