According to Nick Lang and Joey Richter (members of the StarKid theater production company), A Very Potter Threequel is now a real possibility! 

We’ve heard rumors of the third installment in the hilarious Potter musicals (you can watch the first and second by clicking the link and clicking on the respective playlists) however, nothing concrete could be said until they knew the availability of Darren Criss (Harry Potter and writer of the music in the shows). 

Read the following from the linked article, posted 8/28/2011:

The news was learned earlier this evening when Nick Lang said at a charity event that Darren Criss should be available for the musical.

Remember, the key word here is “should”. This isn’t an official announcement but with very little buzz about the show for months, I think fans will cling to this ray of hope!