Just heard from Rob Thomas, and the CW has given "Veronica Mars" a back seven instead of the traditional back nine. This is obviously brand-new information for them, so it'll take a while to figure out how this will affect the later two arcs. (My guess: since they're already at work on arc #2, the last one will take the biggest hit.)
This is good news, certainly for this season, and I don't know that the slightly reduced order is a big danger sign for the possibility of a fourth season. The CW hasn't been as successful out of the gate as expected, and they may be looking to pinch pennies until they get through to next year. Or maybe I'm just being naively optimistic. Wouldn't be the first time.
UPDATE: Rob says he's still trying to figure things out, but as of now, the plan is to do a four-episode arc for the end of the season.