//Shows the current menu on top
  function showstuff(boxid) {
            document.getElementById(boxid).style.zIndex="5";
        }
//Hides the rest of them.
        function hidestuff(boxid) {
            document.getElementById(boxid).style.zindex="3";
        }



<div onclick="showstuff('Menu1'); hidestuff('Menu2'); hidestuff('Menu3');" ;style="cursor:pointer;">