Three scenes were cut that showed aliens disguised as Chinese folks and people in Chinatown getting their memories neuralyzed.
Online in China, people noted that the movie's trailer showed bits that weren't in the final film. What's more, the movie was trimmed down from 110 minutes to 90 minutes. It's a shame they cannot see the film as the filmmakers intended.
This isn't the first Hollywood motion picture that China has censored, and it will not be the last.
In the meantime, be sure to check out i09's MIB3 review.