Ajax-dropdown

I would like to create 3 dropdown-boxes in ajax-script. Select categories in first. When first is populated - select subcategories in second based on first When second is populated - select subcategories in third based on second. I want the select boxes to be automatically populated without any reloading of the page. Just ordinary select-boxes is enough, i will provide you with the tables and the data (mysql) that should be used. Thanks!