Dynamic Drop down list

Hi, Iam using a wordpress theme, called classipress 3.1.5. In my website, there is user input form. user can post Ads using that form. I want to add more features. i need to add a dynamic drop down list (three level) for eg. if the user selects a country from the country drop down list, the state list should be automatically populated in the state drop down list. To make it more clear, if the user selects USA in the first drop down, all the US states should be populated. then if the user selects a state, all the city list within that place should be populated. For this, custom fields can be created easily from back end using wordpress theme.