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>