<div class="contentA">
</div>

<div class="contentB">
  
  <div class="contentC">
  </div>
</div>

.contentA{
  width: 80px;
  height: 80px;
  background: red;
  position: absolute;
  z-index: 2;
}

.contentB{
  width: 100px;
  height: 100px;
  background: blue;
  position:absolute;
  z-index:1;
}

.contentC{
  width: 40px;
  height: 40px;
  background: yellow;
  z-index:3;
  position:absolute;
  bottom:0;
  right:0
}