Alek Skarlatos -- one of the heroes who took down a gunman on a Paris-bound train this summer -- has been invited to Game 3 of the World Series to be honored by Major League Baseball.
TMZ Sports has learned ... MLB reached out to Skarlatos earlier this month and he's all about it! One source close to the hero says he considers it a "once in a lifetime opportunity."
We're told MLB plans to cover the cost of the fight to NYC for the game. No word if he'll be honored before or during Game 3 -- we're told both sides are still finalizing the plans.