Actor Aamir Khan, who is playing a negative role in Dhoom 3, says he has started working on the physical side for the movie, but isnt prepared mentally. While talking to reporters here, the 47-year-old actor said, "I have started preparations for Dhoom 3. For myrole I have began working on the physical side. But I am not mentally prepared yet. In the third installment of hit film series Dhoom, Aamir would be seen as a baddie while Abhishek Bachchan and Uday Chopra would reprise their characters of Jai and Uday respectively. The film, directed by Victor Acharya and produced by Aditya Chopra, also features Katrina Kaif. It is scheduled to hit theatres in 2013.