The men in black are massing.

Sony is moving forward on “MIB 3,” the third pic in its sci-comedy franchise.

“Tropic Thunder” writer Etan Cohen is penning the script, and Barry Sonnenfeld, who helmed the first two films, is said to be attached to  the new  installment, though there is no formal deal or offer as yet.

The studio is eyeing a 2010 start date and could go as soon as the spring.

The X-factor remains Will Smith. The A-lister, who starred  with Tommy Lee Jones in the first two, has not committed to the pic,  though in recent days the buzz in development circles has been that he  is now interested in returning. Smith does not currently have a go  movie lined up.Tommy Lee Jones’ involvement is uncertain.

Sony announced in April that it would bring back the tale of covert, pseudonymous agents who stalk the earth fighting disguised aliens, zapping beasts and memories as they go.

“MIB” has proved to be one of the most successful comedy franchises of all time. The two pics — which came out in 1997 and 2002, respectively — combined to earn nearly $1.1 billion worldwide. The $250m domestic of the first film reps the second-highest total of Smith’s career.

The addition of Cohen, who is said to be deep into his draft, signifies the  studio’s intention to continue the franchise’s top-tier status.  The CAA-repped writer is a hot commodity, penning the sequel to  “Madagascar” and the Will Ferrell-toplined Sherlock Holmes project, thje latter also for Sony, as well as “Thunder.”

Ed Solomon wrote the script for the first “Men in Black,” while Robert Gordon and Barry Fanaro penned the second film.

The WME-repped Sonnenfeld has been concentrating heavily on TV of  late, inking a deal with ABC and ABC studios, and exec producing dark  comedy “Pushing Daisies.” His last feature was the Robin  Williams-starrer “RV.”