function list1(ob){


var id= ob;
        var xmlhttp1;
        if (window.XMLHttpRequest)
            {// code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp1=new XMLHttpRequest();
        }
        else
        {// code for IE6, IE5
        xmlhttp1=new ActiveXObject("Microsoft.XMLHTTP");
        }
            xmlhttp1.onreadystatechange=function()
            {
                if (xmlhttp1.readyState==4 && xmlhttp1.status==200)
                {   alert("ajax1");
                    list2(id);
                }
            }
        xmlhttp1.open("POST","/ajax/listSites.php",true);
        xmlhttp1.setRequestHeader("Content-type","application/x-www-form-urlencoded");
        xmlhttp1.send("id="+id);
        }


              function list2(obb){
        var idd=obb;
        var xmlhttp;
        if (window.XMLHttpRequest)
        {// code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp=new XMLHttpRequest();
        }
        else
        {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
        }
            xmlhttp.onreadystatechange=function()
            {
                if (xmlhttp.readyState==4 && xmlhttp.status==200)
                {   alert("ajax2");
                    list3(idd);
                }
            }
        xmlhttp.open("POST","/ajax/access_points_do_actions.php",true);
        xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
        xmlhttp.send("sel_system_id="+idd+"&action="+'LIST_SLAVE_FOR_ACCESS_POINT');

      }

onchange="Function1(); Function2();"

<select id="se" onchange="javascript:list1(this.value); //this is current code

<select id="se" onchange="Javascript: list1(this); list1(this);">