LaBranche Posts 3rd-Quarter Net Loss  NEW YORK (Reuters) - LaBranche   Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=LAB.N target=/stocks/quickinfo/fullquote"&gt;LAB.N&lt;/A&gt; , the largest  market-maker on the New York Stock Exchange trading floor, on  Monday posted a net loss for the third quarter amid a drop in  principal transcations and commissions.