I think they ARE all in the running. None of them can be discounted this early, unless an unforeseen injury happens (God forbid). If the federation only considers National results, then Zawadzki has a leg up as the defending bronze medalist for two years running. If they also look at the first half of the season, which I think would be a wiser choice, then it's partially dependent on who has better GP results. Nagasu and Gao would probably lead the race.
It'll be too messy to go through and look for each girl's highest-ever PCS, but here are the highest ISU results for each from this season:
Zawadzki:
SP: 27.29 (2012 NHK)
FS: 55.33 (2012 CoR)
Gao:
SP: 28.75 (2013 4CC)
FS: 57.88 (2013 4CC)
Nagasu:
SP: 28.82 (2012 NHK)
FS: 59.14 (2012 NHK)
So, Zawadzki and Gao are both behind Nagasu in this aspect. It's in the FS where this difference is most prominent, but Nagasu also has the disadvantage of URs, whereas neither Zawadzki nor Gao encounter that issue regularly. It should be noted, in all fairness, that NONE of the US ladies can be said to be consistent. Gold is out of the question. Wagner was having a great GP run until the GPF, and then won Nationals by the skin of her teeth. All in all, she fell five times across three consecutive free skates. It was only at WTT, at the very end of the season, that she gave us a much-needed glimpse of "Swagner." Only one lady in the WORLD is currently consistent, and that's Yuna.
Czisny is a shot in the dark right now. We honestly have no idea what form she's in, aside from that video of her FS without jumps (and her spins aren't what they used to be, but what do you expect after 2 hip surgeries?). We'll have a better picture of each girl's chances when the GP rolls around.