Infernal Affairs 3 tells the story about Chan Wing Yan. Chan was being expelled from the school in 1991, SP Yeung Kam Wing graduated, with all police reports distinctions, being the first in whole class. Twelve year later, he was being sent, working to Lau. Ten months after Chan dies, Yeung's spy had blew a case and the spy wish to kill Yeung, but Yeung took his gun and shot him to conclude this failure.
Lau gave Yeung car park seat B3A6 when Yeung met Lau. Yeung asked Chan who he is, but Chan didn't remember him. Sam works with Yeung and Lau is recording Yeung's words so that he can sue Yeung. Yeung is a police SP, which is being sent to Sam as the spy from police. Lau doesn't really know Yeung's Police SP position and wish to catch Yeung to the police head. Yeung work with another inspector who is a good friend of Yeung himself. Lau sneaked into Yeung's office, stealing the cassette, and ask the head to tell him that Yeung is the spy of the triad, Sam's triad.