Edit in JSFiddle

var result = document.evaluate(
  '//div[@id="main"]/p[contains(@class,"content")][3]/a[starts-with(@href,"http://example.com")]',
  document,
  null,
  XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
  null
);

console.log(result.snapshotLength); //1
var elem = result.snapshotItem(0);
console.log(elem.innerHTML);

<div id="main">
  <p class="content">
    <a class="link" href="http://kenjimorita.jp">
    1st Link
  </a>
    <p class="dummy"></p>
    <p class="content">
      <a href="http://example.com/">2link</a>
    </p>
    <p class="content">
      <a href="http://example.com/">3link</a>
    </p>
    <a href="http://example.com/">5th</a>
</div>