Nothing's been decided yet.
Having said that, I truly believe that in the end, NBC will let Community run it's course.
Why? Two simple words:
SYNDICATION RIGHTS.
They may not be making much money right now, but they'll surely make a buttload if they get to that 4th season and can sell off the rights.
Plus with all the positive buzz of the show, one has to wonder about the backlash at NBC for ending it ala the Leno/Conan fiasco albiet at a ridiculously smaller scale.
And this isn't like FOX who cancelled AD or Firefly because unlike them, NBC actually seems to give a **** about how theyre portrayed in the media.