According to the man himself:
Sly had invited me to be in Expendables 2 but I was too busy filming CZ12 and couldn’t make a commitment to the film. But he did extend his invitation to the third movie, which I agreed on the condition that I will be appearing as more than just a minor role with a few scenes.
Sounds great to me. Expendables 2 was a lot of fun, they might as well go all out for the third. Here’s hoping he gets a few decent fights and doesn’t drop out of the movie (literally) like Jet Li did in Expendables 2.