Aetna Earnings Rise as Membership Rises  CHICAGO  (Reuters) - Health insurer Aetna Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AET.N target=/stocks/quickinfo/fullquote"&gt;AET.N&lt;/A&gt; on  Thursday posted a sharp rise in third-quarter profit as it  boosted membership and benefited from a big tax refund from  discontinued operations.