The Series Was Wrapped Up, Don't See How A Third Series Is Likely
First off, I would loooooove more "Psychoville".
The trouble is, everything was cleared up, and the David's mother bit was really not something that calls for a third run. All tales on this scale usually end on something up in the air, a "what if". This is no different.
Everything's out in the open now. We understand it all. Almost all the characters are dead. What's left?
With the poor ratings, the BBC would be keen on dropping the show. Reece Shearsmith and Steve Pemberton did express interest in doing a third series, but I could see them both just wanting to move on.
The BBC didn't put a lot of effort in promoting this series. Reece and Steve didn't even get word of a finale airdate until the last minute, and they had wrapped up filming months ahead. The move to Monday night for the series finale was unprecedented. Even Reece Shearsmith didn't understand why they did it, but said flat out that they must want to kill off any viewers they had left. Networks do this all the time to get rid of series. Seth MacFarlane ("Family Guy") had a name for this. They did it with his show.
Whatever happens, I'm sure Reece and Steve would likely do an all-new series down the line.