#caja1{
  position:absolute;
  background-color:red;
  left:20px;
  top:20px;
  width:300px;
  height:300px;
  z-index:1;
}

#caja2{
  position:absolute;
  background-color:green;
  left:100px;
  top:100px;
  width:300px;
  height:300px;
  z-index:3;
}

#caja3{
  position:absolute;
  background-color:blue;
  left:60px;
  top:60px;
  width:300px;
  height:300px;
  z-index:2;
}