A multi-billion dollar deal helped fuel third-quarter earnings growth for Aetna Inc., but the performance missed expectations, and shares slid Tuesday after it became the latest big health insurer to warn about challenges that lie ahead. Chairman and CEO Mark Bertolini told analysts that the Hartford, Conn., insurer is committed to growing operating earnings next year, and it expects the floor for its performance to be around where it finishes this year. But Aetna also expects to pay about $600 million toward a health insurance fee required as part of the health care overhaul, and it faces a Medicare Advantage funding cut also mandated by the law, which aims to fund coverage for millions of uninsured people over the next few years.