selecting list item

hi guru's

on page ive three list box's .
what is need when i select a item from a first listbox values depending on the selected value inserted in the second listbox and agn based on the second listbox values third listbox updated.

i m using for that is
i m changing the listbox.selectedlistindex=0 of first listbox and i write the code on the first listbox's selectedindexchanged event which generate the second listbox and set as listbox2.selectedindex=0.

but listbox is not generating i need to click the listbox item to generate the next listbox list.

if you get my point pls tell me wht should i do.

thnx