3 Level Dynamic MySQL Dropdown

I'm trying to make 3 drop downs (based off of mysql tables), to narrow down the selection. Source, School, and Program. So when when they select SOURCE(source), all the SCHOOLs in drop2 will be based on the SOURCE selected in the previous dropdown, and whatever school(cid) is selected, the program will be based off that school(cid) and display all the programs for that school. I've google'd high and low for something, and no avail. Here is what I have, I am just trying to make 3 drop downs narrow in, without pressing submit (I am new to ajax and javascript etc) The issue may be in that java part, I've look in the php and mysql, but nothing.

PHP Code:
<table width="517" border="1"> <tr>
<td width="87">Source:</td> <td width="414">
<? echo'<select name="source" onChange="showUser(this.value)">
<option value="">--Select--</option><?php while() { } ?>'; .....

My javascript
Code:
var xmlhttp;
function showUser(str){
xmlhttp=GetXmlHttpObject();
if (xmlhttp==null){
alert ("Browser does not support HTTP Request");
return;
} .....

Right now, the results display in a table, I don't want that - I need them to display as results in the next dropdown.