*This wasn't really planned, but since the show's seasons (series, whatever) are so short, I'm just going to go ahead and review all three episodes of Sherlock this year. It'll be fun.*
At a certain point, Sherlock was going to have to shake things up. I noted in my review of "The Empty Hearse" that the show was trying new things this year, as that episode placed more emphasis on the Sherlock-John relationship than it had in the past, with the plot-driving mystery sort-of taking a backseat. It was an important step for the show to make: after spending nine hours with Sherlock prior to "The Empty Hearse," the audience needed reason to believe that the self-described "high-functioning sociopath" wasn't just a brilliant asshole, but capable of having some form of human connection, even if it is basic and juvenile.
"The Sign of Three" was an extension of that focus on Sherlock's relationship with John, with the episode unfolding over the course of John's wedding to Mary. Sherlock has been named best man, and much of the episode consists of Sherlock's speech at the reception, in which he manages to insult John and the majority of the other people in the room, turn around and have his most human moment to date in explaining how much John means to him, regales the audience with two seemingly-unrelated stories of mysteries the two have recently worked on, then realizes that a murder will occur before the end of the day. That's a lot of things happening for an episode where all that much isn't really happening.
Once again, those mysteries take a backseat to the episode's emotional core and humorous tone. It's worth noting that, in the show's previous two seasons, the middle episode has often been the weakest of the three. However, the show seems to have figured out how to resolve that problem, and the structural experimentation is a major reason for this episode's success. For one, it finds a way to take the basic structure of the Sir Arthur Conan Doyle tale it is adapting, The Sign of the Four, and implement it in an interesting and exciting way. By taking two separate cases and connecting them through Sherlock's retelling of the events, the show finds a new angle for presenting the mystery that sets it apart from the preceding episodes. That's not to say that there was something inherently wrong with the way a standard Sherlock episode is structured; procedurals, by nature and name, do follow a certain procedure in their storytelling, and Sherlock is a gussied-up procedural at heart. But off-format episodes do help engage the audience in a new way, which makes for a more enthralling 90 minutes of television.
But once again, the mystery isn't really the heart of this episode, though it was much better integrated into the main plot than "The Empty Hearse"'s terrorist attack. Instead, we get a much better glimpse into how Sherlock is reacting to the change that John's marriage will bring to their relationship, and his deep-seated anxiety about that. I noted at the beginning of this piece (and in last week's review) that any humanizing of Sherlock is welcome, even if it does seem childish at times. Doyle's Sherlock is, in many ways, a proto-anti-hero (stay with me). He was brilliant, yes, but he was also an anti-social, abrasive addict who had trouble connecting with others and harbored an obsession with his nemesis Moriarty that eventually ended in his death. Though Sherlock was ultimately on the "right" side of the moral spectrum, he was a separate type from the majority of literary heroes.
The show, then, has successfully shepherded the character into the modern era, specifically the age of the television anti-hero. However, what makes an anti-hero effective as a character is that no matter how bad they behave, how hazy their morality is, they are still recognizably human and capable of feeling the consequences of their actions. This was a hallmark of Tony Soprano, as he wrestled with his guilt in his dreams and therapist's office, and, more relevantly, of Dr. Gregory House, who balanced his brilliant medical diagnoses with his Vicodin addiction and irascible attitude (House was loosely based on Sherlock Holmes), at least in the first couple of seasons of House. Through the first six episodes of Sherlock, however, the show hadn't offered much in the way of humanity in Sherlock, making him a fun character to spend an hour and a half with, but as the time accumulated and his dickish behavior became more hurtful to those around him, it became harder to really root for the guy. And given the way "The Reichenbach Fall" ended, and Sherlock's subsequent return in "The Empty Hearse," for him to continue being terrible to everyone would have been disastrous to the audience reception of the character.
It's to the show's credit that they've found a way to make this influx of humanity organic to the relationship of the characters, rooting Sherlock's surprisingly gushy speech in the history between the characters that the audience has been privy to. That, ultimately, is what makes "The Sign of Three" such an unusual and excellent episode of Sherlock. The mystery is there, yes, but this is the first episode to truly spend the bulk of its running time celebrating the characters, let them bounce off each other, and ultimately give the audience greater insight into who they are and what they're capable of.
P.S. All of this and I didn't even mention that there's a whole segment of this episode where Sherlock and John are very drunk and try to solve a case where a woman believes she went on a date with a ghost. It's the most sitcom-y thing this show has done yet, but it was really fun.