connect 3 combobox

Greetings experts,

I'm beginner in AS3 and I want to make a JOB Classification Application by changing from frame to another in one and same scene using combobox. every frame has a data.

I made 3 comboboxes and success to connect combobox1 to combobox2 by write actions of moving from frame to frame as writed below, but I failed to continue in connecting the combobox2 to combobox3. Can anyone help please?

I used simple actions here as a rookie prograammer:

import flash.events.Event;
import flash.events.MouseEvent;
import fl.controls.ComboBox;

combobox1.addEventListener(Event.CHANGE, changeFrame);

function changeFrame (event:Event):void
{
    if (combobox1.selectedItem.label == "General Employee")
    gotoAndStop (2);
    else if (combobox1.selectedItem.label == "Education Employee")
    gotoAndStop (3);
    else if (combobox1.selectedItem.label == "Health Employee")
    gotoAndStop (4);
}

combobox2.addEventListener(Event.CHANGE, changeFrame1);

function changeFrame1 (event:Event):void
{
    if (combobox2.selectedItem.label == "Employee")
    gotoAndStop (5);
    else if (combobox2.selectedItem.label == "Semi-Employee")
    gotoAndStop (6);
    else if (combobox2.selectedItem.label == "Contracted")
    gotoAndStop (7);
    if (combobox2.selectedItem.label == "Teacher")
    gotoAndStop (8);

    else if (combobox2.selectedItem.label == "Adminstrator")

    gotoAndStop (9);

}

combobox3.addEventListener(Event.CHANGE, changeFrame2); 

function changeFrame2 (event:Event):void
{
    if (combobox3.selectedItem.label == "1st Level degree 1")
    gotoAndStop (10);
    else if (combobox3.selectedItem.label == "1st Level degree 2")
    gotoAndStop (11);
    else if (combobox3.selectedItem.label == "1st Level degree 3")
    gotoAndStop (12);
}

and so on like this,

The problem is: the second combobox didn't work and go to the 3rd combobox in choosed frame, why?

Also this error appears:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

at prog2_Scene2_fla::MainTimeline/frame1()[prog2_Scene2_fla.MainTimeline::frame1:49]

*Second Part:

In the end of choices the is a number appear (Num1), I want to put this Num1 in Math actions, e.g: Num1 * 30% + Num2 = output field (RESULT), How can I make this?

Thanx in advanced

Ritchy GH