Having now watched the entire INFERNAL AFFAIRS trilogy, I've concluded that only the original film was necessary. The first sequel took the form of a prequel, an entirely extraneous piece of back story that adding nothing to the original and would have worked better as a stand alone movie. This, the second sequel, is even worse, a muddled attempt at tying together both prequel and sequel, adding in lots of entirely superfluous stuff and additional characters that are all entirely necessary.
Tony Leung returns to the movie series, but his scenes amount to nothing more than padding; his character was so carefully delineated in the first movie, there's entirely nothing to add. Andy Lau has the most interesting role in a sub-plot that follows on directly from the first film's, and this section of the film was the most entertaining. But it's still rather unnecessary, and would have been much better had it been tied up at the end of the first film, as Scorsese did when he remade the series as THE DEPARTED.
Overall, INFERNAL AFFAIRS 3 feels confused and muddled. The constant jumping between past and present happens so much that it's ridiculous, and it becomes overwhelming clear that this was only rushed out to make a quick buck or two after the success of the original film.