<script type="text/javascript">

        function check(obj) {

            var checkboxs = document.all.checkboxs;

            if (obj.checked==true) {

                for (var i=0;i<checkboxs.length;i++) {

                    if (checkboxs[i]!=obj && checkboxs[i].checked==true) {

                        checkboxs[i].checked=false;

                    }

                    

                }

            }

        }

    </script>

<input type="checkbox" name="checkboxs" onClick="check(this)">

<input type="checkbox" name="checkboxs" onClick="check(this)">

<input type="checkbox" name="checkboxs" onClick="check(this)">