<style>
div{
  background-color:white;
  top: 0px;
  bottom: 0px;
  left: 0px;
  right: 0px;
}
</style>
<div style="position: fixed; z-index: 2;">
   div 1
   <div style="position: fixed; z-index: 3;">
     div 2
   </div>
 </div>
<div style="position: fixed; z-index: 2;">
 div 3
</div>