No duplicates in three dropdowns

Hi,

I've picked up some very useful JS on the WEB (free to use) which prevents
duplicates in a second pulldown if has been selected in the first and
vice-versa. The code is at
http://javascript.internet.com/forms/no-duplicates.html

The problem is I want to extend this over three pulldowns but don't know how
to do it.

Can anyone help?