Jackass co-star Steve-O announced on "The Howard Stern Show" that a third Jackass movie is in the works, according to /film:

"I was told to rest up for Jackass 3 which will start shooting in January," said Steve-O.
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Steve-O is currently starring in his own reality show on USA Network, Dr. Steve-O.

Dont't forget to also check out: Jackass: The Movie, Jackass Number Two, Jackass