I actually forgot this game existed until recently. Black Mirror II ends pretty definitively (or so it seems). That said, I'm glad i rediscovered it and decided to play, as the Black Mirror series are some of my favorite point and clicks and III adds a lot to the series.
For one thing, there is a marked difference, in my opinion, in how coherent the storyline is. The first game was interesting, but sort of all over the place - if there's one thing that forcing a more linear rigidity does give to a game, it's the gift of coherence. With II becoming more linear, it was slightly more understandable what you were doing at any given time and why, and this one seems to have really maxed that trait out.
This is a very interesting and dark point and click horror game that fans of the genre will really enjoy, whether or not they have played the first two (it does a good job of introducing characters and events previously used in the series without being too hamfisted.) The locations are fun to explore and the plot has lots of depth. If you're into history, family dramas, insanity studies, or atmospheric creepers you'll have a good romp with this one. It's also a long game, with many things to poke around in, so you're likely to be entertained for a while