Drop Down List Value From Multi Source

hi

i need to display a drop down list which get its value from 3 other drop down list.

echo CHtml::dropDownList('stclass', '', array(), 
  array(
    'prompt'=>'Select Class',
    'ajax' => array(
    'type'=>'POST', //request type
    'url'=>CController::createUrl('Assignment/getStudentByClass'), //url to call.
    'update'=>'#'.CHtml::activeId($model,'studentID'), 
)));
			
echo CHtml::dropDownList('sectionID', '', CHtml::listData(Usersectionsecurity::model()->findAll(), 'Section_Code', 'E_Section_Name'), array('empty'=>'Select Section'));

echo CHtml::dropDownList('majorID', '', CHtml::listData(Usersectionsecurity::model()->findAll(), 'Major_Code', 'E_Major_Desc'), array('empty'=>'Select Major'));
					
echo $form->dropDownList($model,'studentID', array(), array('empty'=>'Select Student'));

to display the student correctly, i have to select the class, the section and the major

i dont know how to write this code

can anyone help me

thank you