There probably is a third Riddick movie coming At least Diesel said so
here's the article I found:
"In an interview with GameSpot talking about the upcoming "The Chronicles of Riddick: Assault on Dark Athena" game, Vin Diesel hinted at a third movie in the "Chronicles of Riddick" franchise.
"Maybe we're too tight-lipped about the next 'Chronicles of Riddick' film, and I think circa the release of [Dark Athena] that's probably when you'll start hearing more about the next 'Riddick' film," Diesel said. "It is underway and I almost think it's a coincidence that we haven't heard anything, that there hasn't been a lot of public stuff on the 'Riddick' movie.
Diesel added that David Twohy, who wrote and directed Pitch Black and The Chronicles of Riddick, is finishing up the script."