position: fixed

div-one

div-two

div-three

HTML

<div id="one">div-one
  <div id="two">div-two</div>
</div> 
<div id="three">div-three</div>

CSS

#one {position: relative; background-color: #0000FF;}
#two {position: fixed; top: 0px; left: 0px; background-color: #00FF00;}
#three {background-color: #FF0000;}