The Welsh-born star, who is soon to be seen in Batman Begins sequel The Dark Knight, has already started shooting the first of those films, Terminator Salvation: The Future Begins.
"He's really an actor's actor, so we're very proud," said Victor Kubicek of film company Halcyon.
"Christian was our first choice and he's a big fan of The Terminator, so we're very lucky," Kubicek told the BBC at the Cannes Film Festival.
"But with Batman he's already done the whole franchise thing, so we weren't sure he'd respond."
Luckily, said Kubicek's business partner Derek Anderson, "he read the script and he loved it, so he's signed on for all three."
The first new installment, co-starring Sam Worthington, Anton Yelchin and Moon Bloodgood, centers on John Connor, now in his 30s, who leads what is left of the human race in a battle against the machines. Directed by McG, Warner Bros. will release the film on May 22, 2009.