No letters needed
Mooseflyer,
I've been to several of these Air Inc. seminars. All you need to bring is your resume with no cover letter required. Make sure you have a well formatted resume.
Of the three you mentioned, the only one that attending the job fair helps with potentially getting an interview with is Alaska. I'll admit though that I know very little about them. They backed out of the last one late.
JB won't even take your resume, but they may ask for your name on a 3x5 card to show you were there (Jan. seminar they didn't want or take anything). They are just there to answer questions. Very nice folks but glad handing won't give you a leg up. I believe SWA is now following the same path.
On the other hand, if you are interested in AirTran or ATA, if you don't have an internal recommendation, this is the only other way to get an interview. I know for a fact this works. My former boss and another O-5 from my wing both got calls from AirTran (last August after June seminar).
Also, expect that some on this board will trash Air Inc for various reasons. However, as a military guy with limited Airline contacts (me anyway), it's a great way to get started.
Best of luck,
Catfish