Infernal Affairs 3 builds cleverly on the plotline of the first movie, but with its complex story and frequent switches between past and present, is likely to seriously confuse anyone who comes to it without having seen Part 1 first to understand the two main characters. For those who have, this film brings out further details of the relationship between the two, superbly played again by Tony Leung and Andy Lau. With frequent flashbacks, the film focuses on extending the story of Triad mole Ming (Andy Lau), warping up the tension as the stresses of his double life become intolerable. Leon Lai's usual expressionless performance, which mars his other films, works well here as it leaves you few clues about his character's motivation until the climax.
If you enjoyed Part 1, you will enjoy this. (I haven't seen Part 2 yet.)