.slice & .wrapAll

HTML

<div id="nav">
<p>"Test this theory"</p>
</div>

CSS

.test {
    color: red;
}

JavaScript

// wraps last 3 li's of #nav with span that has class .test
var group = $( "p" ).text().slice( 0, 1 );

$(group).wrapAll( "<span class='test' />");