chromium/chrome/test/data/extensions/iframes/main.html

<!doctype html>
<html>
  <title>Main</title>
  <body>
    <iframe name="frame1" src="frame1.html"></iframe>
    <iframe name="frame2" src="frame2.html"></iframe>
    <iframe name="frame3" src="frame3.html"></iframe>
  </body>
</html>