Here's a JS Fiddle for you, it only uses one single div to work.
#shadowBox {
background-color: #ddd;
margin: 0px auto;
padding: 10px;
width: 220px;
box-shadow: 0px 8px 10px gray,
-10px 8px 15px gray, 10px 8px 15px gray;
}
You set a shadow on the bottom, bottom left, and bottom right. With soft shadows it gets a bit tricky but it is doable. It just needs a bit of guesswork to decrease the middle shadow's blur radius, so that it looks seamless and not too dark when it overlaps with the side shadows.
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