Related combobox

      i have 3 comboboxes, i want to load 2nd combobox based on the
value selected in 1st combobox. and 3rd combobox should be loaded
depending on the value selected in the 2nd combobox.
      i have used observe field for 2nd and third combobox , it works
for the first combobox but not the second one.

This is often referred to as "chained pickers", you might have better google-luck with that term.