#caja1{
    position:absolute;
    background-color:red;
    left:10px;
    top:10px;
    width:200px;
    height:200px;
    z-index:1;
  }
  
  #caja2{
    position:absolute;
    background-color:green;
    left:50px;
    top:50px;
    width:200px;
    height:200px;
    z-index:3;
  }
  
  #caja3{
    position:absolute;
    background-color:blue;
    left:30px;
    top:30px;
    width:200px;
    height:200px;
    z-index:2;
  }