CodePen Embed - CSS z-index

<div class="box"></div>
<div class="box"></div>
<div class="box"></div>

body{
  margin: 0;
}

.box{
  position: absolute;
  width: 150px;
  height: 150px;
}

.box:nth-child(1){
  background: green;
  z-index: 1;
}

.box:nth-child(2){
  background: red;
  width: 100px;
  height: 100px;
}

.box:nth-child(3){
  background: blue;
  width: 50px;
  height: 50px;
}

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