HTML:
<div class="parent">
<div class="one"></div>
<div class="two"></div>
<div class="three"></div>
</div>
jQuery
parentWidth = $(".parent").outerWidth(true);
oneWidth = $(".parent .one").outerWidth(true);
twoWidth = $(".parent .two").outerWidth(true);
$('.parent .three').width( parentWidth - oneWidth - twoWidth);
But the thing is, either DIV .one or .two may not exist some times, how do I modify the jQuery for it?
Thanks