U.S. Drugmakers Hurt by Competition  NEW YORK (Reuters) - Merck   Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt; and two other U.S.  drugmakers posted weak third-quarter results on Thursday, hurt  by intensifying competition and the loss of sales from drugs  found to be potentially dangerous.