<script language="javascript" type="text/javascript">
var indice = 1;
function mostrar(div)
{
document.getElementById(div).style.zIndex = indice++;
document.getElementById(div).style.display = 'block'
}
</script>
<style type="text/css">
#cont {position:relative; top:0; left:0;}
#cont > div {display:none; height: 100px; width: 100px; position:absolute; top:0; left:0; }
</style>
</head>
<body>
<a href="#" onClick="mostrar('div1')">1</a>
<a href="#" onClick="mostrar('div2')">2</a>
<a href="#" onClick="mostrar('div3')">3</a>
<div id="cont">
<div id="div3" style="background-color:blue;"></div>
<div id="div2" style="background-color:green;"></div>
<div id="div1" style="background-color:red;"></div>
</div>