Edit in JSFiddle

<div class="relative">
    <div id="div1">div1</div>
    <div class="absolute">div2</div>
    <div id="div3">div3</div>
</div>

.relative {
    position: relative;
}

.absolute {
    position: absolute;
    background: red;
}

#div1 {
    background: #ccc;
}

#div3 {
    background: blue;
    height: 2em;
}