[monolithed]

Думаю так:

<script type="text/javascript">
window.onload = function(){
    var form = document.forms[0].elements, i = form.length;
    while(i--) {
        form[i].onclick = function(){
            form[form.length-1].value = this.type != 'text' && this.checked ? this.name : '';
        }
    }
};
</script>

<form action="" method="">
    <input type="checkbox" name="name1" />
    <input type="checkbox" name="name2" />
    <input type="checkbox" name="name3" />
    <input type="text" value=""  />
</form>