Add this code to the head of your form. Edit the #item1_text_1 and the #item3_select_1 to fit your forms input id.
<script src="common/libs_js/jquery-1.4.4.min.js"></script>
<script>
$(document).ready(function () {
$('#item3_select_1').change(function() {
$('#item1_text_1').val(
$('#item3_select_1').map(function() {
return $(this).val();
}).get().join(',')
);
});
});
</script>