Update template variable (Dropdown List menu)

Hello,
I have 3 dropdown (template variable) that the first have all the continent of the table of the modx database and the second the country and the third all the region.
What I want it’s when you select one continent, show in the second combobox all the contries of this continent and when you select the country show in the third combobox all the region for the country selected

How I get the value selected for I update the next template variable(combobox) ?

How I update the template variable with the last value selected?
I need to know how I get the value selected
I see this API modx getTemplateVarOutput($idname=array(), $docid="", $published="1");
but I don’t know how I made
I have this query: @select country, id_country FROM country WHERE continent=getTemplateVarOutput($idname=array(), $docid="61", $published="1");

I don’t know if it is correct... but doesn’t work

how I update the next variable with the value selected?

I do not know if I could explain

I create three template variable of the type DropDown
The three variable contain values od the database.
And I want that when you select one value on the first variable (continent), the next variable could show all the values that correspond of the first selection (country) and the same for the thirt variable, when you select a value of the second combobox the third show all the values that correspond of the second selection (region).

All the values are on the different tables of the database

How I update automatically the variables?

I don’t think you can do this unless you write some clever javascript code to manipulate the TV editing screen.