Oscar history hasn’t been kind to sequels, let alone “threequels.” The third installment of James Cameron ’s Pandora saga arrives in movie theaters in December with technological marvels and box office expectations that would make most studios salivate. But when it comes to Academy voters, “ Avatar: Fire and Ash ” faces a challenge no amount of motion-capture innovation can solve: the “been there, done that” problem. American Cinematheque to Program Westwood's Village Theater, Aiming for 2027