Menu and its sub-menu in the submit form

Category menu and its sub category menu in the submit form

Hi dear,

I would like to get three level category information from the user and save
them to DB.

So, I inserted the 1st level category in the submit form using Insert > Form >
List/Menu option for users to select an item among items of the 1st level
category.

Then, I don't know how to insert 2nd level category menu depending on the item
selected in the 1st level category menu and how to insert 3rd level category
menu based on the item selected in the 2nd level category menu.

For example)
1st level menu: country name
2nd level menu: state or province name
3rd level menu: city name

If the user selects USA in the 1st drop down menu, the names of the states
should be loaded in the 2nd drop down menu as items to select and if the user
selects Canada in the first level menu, then province?s names should be loaded
in the 2nd level menu, and so on.

Please, let me know how to solve the problem.

Have a nice day


Robert