When using the cascading dropdown functionality the value is stored in a single line of text field for each of the levels of the cascading dropdown.
To get the value you must use something like this:
function callMeOnChangeOfYourField(){
var level1 = getFieldValue("Level1FieldInternalName");
var level2 = getFieldValue("Level2FieldInternalName");
var level3 = getFieldValue("Level3FieldInternalName");
alert(level1 + " - " + level2 + " - " + level3);
}
Change the field names to match your fields and call the function from a rule triggering on change of the last field in your cascading dropdown.
Alexander