Viacom Posts Third-Quarter Net Loss  NEW YORK (Reuters) - Viacom Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=VIAb.N target=/stocks/quickinfo/fullquote"&gt;VIAb.N&lt;/A&gt;&lt;VIA.N&gt; on  Thursday posted a third-quarter loss as a result of charges  from discontinued operations and a writedown of its assets, but  revenue rose on advertising sales growth at MTV and Nickelodeon  cable networks and the CBS television network.