Patrick Dempsey has revealed he’s joining the cast of Paramount’s “Transformers 3.”Michael Bay returns to direct the third installment, reuniting with franchise stars Shia LaBeouf and Megan Fox. Film unspools July 1, 2011. Dempsey told reporters in Brazil that he begins lensing the third outing mid-summer. He also said in the interview that he’ll play a darker character. Dempsey, star of ABC’s “Grey’s Anatomy,” was last in theaters with romantic comedy “Made of Honor.” Other bigscreen credits include “Iron Jawed Angels” and “Enchanted.” He’s repped by UTA.