Robert Downey Jr. set to appear as Iron Man in 'Captain America 3'

Robert Downey Jr. is on the verge of signing on to “Captain America 3,” with Tony Stark’s Iron Man set to play a key role in bringing the Civil War storyline from Marvel’s comicbooks to the bigscreen and trigger the start of a new phase of movies from Marvel Studios.

The actor is in negotiations to play the billionaire in the yet-to-be-titled third installment, that is slated to begin production in the Spring for a May 6, 2016, release. Downey suits up as the superhero in next summer’s “The Avengers: Age of Ultron” as part of a previous deal that also calls for him to reprise the role in “The Avengers 3.”

Tony Stark and Steve Rogers have never really gotten along. They may not be enemies, but if not for The Avengers they probably wouldn’t be friends, and the abrasive relationship between Iron Man and Captain America has fueled one of the greatest storylines in the modern Marvel Comics universe: Civil War, the epic 2006-07 crossover series that pitted hero against hero.

They feud over the Superhero Registration Act, which forces anyone with superhuman abilities to reveal their identities to the U.S. government and agree to act as a police force for the authorities.

Downey has repeatedly stated he is happy to continue playing Iron Man as long as Marvel — which launched its enormous cinematic franchise off his charismatic hero’s metallic back — continues to share the wealth. The actor was listed by Forbes as the highest-paid actor in the world, having earned $75 million from his backend deal for Iron Man 3, the highest-grossing live-action movie of last year at $1.21 billion.

Brothers Joe and Anthony Russo, who directed Captain America: The Winter Soldier, are already on board to direct Captain America 3. Click the Movie button below for more info: