According to the site, Jemaine Clement willl play Yaz:
Flight of the Conchords stars Jemaine Clement and Bret McKenzie declined to return for a 3rd season of the HBO series, but they have been touring around the world and selling out most venues. Which bodes well for Jermaine now that he’s tuning up to play the villainous Yaz in Men in Black III, the sequel that is being fast-tracked by Columbia Pictures. Will Smith stars with Tommy Lee Jones and Josh Brolin, and Barry Sonnenfeld directs. Clement will next be seen co-starring with Steve Carell in Dinner For Schmucks, the Jay Roach-directed DreamWorks comedy that Paramount distributes July 23. Both films are produced by Walter Parkes and Laurie MacDonald, who saw what work Clement was doing on Schmucks and lobbied for him to join MIB3.
As they say, Men in Black 3 is being fast-tracked so expect more details like this one to start emerging in the coming weeks.