Shuttle America has a base in ATL.
Chautauqua has a base in GSO.
Republic has a base in DCA.
When you interview, you have the opportunity to tell them which certificate you'd like, and why. With the way we're hiring right now, I imagine they'd be more than happy to accomodate you if you had a good reason for wanting one airline over the other. When you get the job offer and class date, they tell you which airline it's for so you'd have the chance to turn the offer down if it wasn't what you wanted.