hai

try this code:

 <script type="text/javascript">
        function ValidateContactCheckBox(source, args) {
            debugger;
            var checkbox = $('#<%=chkContact.ClientID %> input[type=checkbox]:checked');

            for (var i = 0; i < checkbox.length; i++) {
                if ($(checkbox[i]).next()[0].innerHTML >3) {

alert('Maximum of three checkboxes only can be selected');

                 args.IsValid = false;                   

                }
            }

             args.IsValid = true;
                    return;          

        }
    </script>