Go with Navy suit, white shirt red tie and black polished leather shoes. That suit will work at all three airlines! Maybe a good tie at United but the everything else would work.

One guy in my interview group in Jan was well qualified and his dad was a 30 AA Capt in DFW. He didn't pass the interview. He was a nice go too but the only that made him standout was he wore a suit that I would wear in NY or on Wall Street. He looked well polished but stood out too much I thought. Maybe that had nothing to do with him not getting the job but AA has the highest hire rate once you make it to the interview so it had to be something.
Good luck.
My buddy who is a fighter guy just finished the online interview Tuesday. Hopefully he gets an interview soon!