Their source is unclear to say the least, but BD believe they have exclusive word that Karl Urban has signed on to Riddick 3, to reprise his Chronicles role of Vaako.
Vaako, you'll recall, was commander of the all-conquering Necromongers, tasked last time by the Lord High Marshall with bringing down Vin Diesel's Riddick and returning with his head. He was married to Thandie Newton, the lucky devil, and had designs on the LHM's throne, but [Chronicles spoiler] was prevented from assuming power by the shiny-eyed Richard B, who won the right to lead the Necromonger troops because, "You keep what you kill" and he killed their big boss.
Urban's return isn't a great surprise, since various scripts featuring Vaako have surfaced along the road to Riddick's production. The size of that role has yet to be revealed, however. The screenplay monikered Dead Man Stalking was said to feature Vaako heavily eighteen months ago, but more recent reports of the casting sheet seemed to indicate something more along the lines of a cameo.
Either way, whatever you think of Chronicles, it's good to see the series at least keeping some continuity, rather than opting for an embarrassed reboot. The leaner, meaner, more-along-the-lines-of-Pitch-Black Riddick threequel is gearing up for an imminent shoot, with a release planned for sometime next year. As before, it's directed by David Twohy.