Merrill Profit Falls on Lower Trading  NEW YORK (Reuters) - U.S. investment bank Merrill Lynch    Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MER.N target=/stocks/quickinfo/fullquote"&gt;MER.N&lt;/A&gt; on Tuesday said third-quarter profit fell 8  percent, hurt by reduced trading in debt and equity markets  which pushed revenue below expectations.