Посмотрите, что решение, показанное здесь, работает хорошо.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<title>JS Bin</title>
</head>
<body>
CourseList
<select id="CourseList">
<option value="1">1</option>
<option value="2">2</option>
</select>
GroupList
<select id="GroupList">
<option value="1">1</option>
<option value="2">2</option>
</select>
SeriesList
<select id="SeriesList">
<option value="1">1</option>
<option value="2">2</option>
</select>
<script>
$(document).ready(function(){
$('#CourseList,#GroupList,#SeriesList').on('change',function(){
var f = document.createElement("form");
var i = document.createElement("input"); //input element, text
i.setAttribute('name','Code_Course');
i.setAttribute('value',$('#CourseList').val());
f.appendChild(i);
var i = document.createElement("input"); //input element, text
i.setAttribute('name','Code_Group');
i.setAttribute('value',$('#GroupList').val());
f.appendChild(i);
var i = document.createElement("input"); //input element, text
i.setAttribute('name','Code_Series');
i.setAttribute('value',$('#SeriesList').val());
f.appendChild(i);
$.ajax({
type:'POST',
url:"page2.php",
processData: false,
contentType: false,
data: new FormData(f),
success:function(html){
$('#table').html(html);
}
});
});
});
</script>
</body>
</html>