From WENN.com

The martial arts legend was unable to appear in the second installment due to prior commitments but Stallone later extended the invitation for the third film.

Chan recently vowed to step away from action movies, so he asked the Hollywood tough guy to re-work the part and give him a more comedic role, according to Malaysian film portal Cinema Online.

He tells the website, ""Sly (Stallone) had invited me to be in (The) Expendables 2 but I was too busy filming CZ12 (Chinese Zodiac) and couldn't make a commitment to the film. But he did extend his invitation to the third movie, which I agreed (to) on the condition that I will be appearing as more than just a minor role with a few scenes.""