The show returns to BBC One on Wednesday 1st January at 9pm with The Empty Hearse, written by Gatiss. Before then, though, a prequel entitled Many Happy Returns will go live on BBC iPlayer on Tuesday 24th December.
The new series will feature Freeman's real-life partner Amanda Abbington as Watson's love interest Mary Morstan, plus Lars Mikkelsen as the main villain Charles Augustus Magnussen. It will also see Gatiss returning as Holmes's brother, Mycroft.
It's always a kind of cold-faced thrill because you get the first enjoyment of it. It's like being the first audience of the finished product, so it was a real treat. We knew what the rough trajectory was over the three films, but when you get the full script it's just a delight. The first thing I go to is the big scenes, and every one of those scripts is a terrific read. It's such a joy to read them for the first time - you get that thrill that hopefully the audience will get when they sit down and watch it.
What was it like getting back into character after a two-year break?
I would say it was easier than the second series, but it's never completely easy because of the break and because of how extraordinary a character he is. No matter what comparisons people draw between me and him he is very different in his temperament, he is more mercurial, his rhythms are faster and more aggressive than mine are, and that goes for his intelligence and speed of talking - I feel like I have to ramp up a gear, which takes a bit of time to get used to. But you know, I love it, and however hard it gets, and it is a tricky one to pull off, it's a character I love playing and I always feel sad saying goodbye to him.
What do viewers have in store for series 3?
Without giving too much away there is a fantastic trajectory in Sherlock's character arc which is going from a position of not really fitting in and then doing incredibly well and forming a bond again with John.
They have some extraordinary adventures in the first and second episodes and then in the third they are challenged by a situation and a master villain who brings him to his knees. What's exciting about this series is we see Sherlock in real peril. We know from the end of series two he staged his death, he was out of the game but calculating every move and therefore in charge. With this challenge he really is out of control. He really does lose his authority on the situation and it's really exciting to see a hero in that much jeopardy and it's very interesting to see what it does psychologically to him.
As far as plot goes, Sherlock and John reunite, there is an explanation and there is a new character in the shape of Mary Morstan - it's really about how the three of them kind of co-exist. It doesn't become a trio, but she is a leading figure in it. She's not just a stay-at-home wife, she is someone who is very involved in both of the boys' realities, but it's a wonderful new dynamic to play with.
What do you think Lars Mikkelsen brings to the series?
An incredibly steely reality which is harrowingly possible. He is a businessman - he's not a mad chaotic villain, not like the award-winning performance Andrew Scott gave as Moriarty. He is something of great calm and measure that has a depth to it which is an even bigger challenge than the chaos of Moriarty. He is so measured and precise - he is like a shark, a terrifyingly perfect predator of our age.
He will chime with people who know what a bully is - he is the archetypal bully, and his means and methods are chillingly everyday in our culture. He's a smart creation, very real and not fantastical. He's got a foreign perspective on the English culture which Holmes so perfectly personifies and the fact that he fights on the side of the angels. He sort of reduces what Holmes protects.
Do you think Sherlock will understand John's reaction to the news that he is in fact alive?
I think he completely thinks that John will understand what he has done, and be fine with it. I think he is expecting a wry smile, a handshake and maybe a laugh or two and then off on a case. He gets it so wrong, so so wrong. He's rusty, he regresses and it takes him a little while to get on form, to smell out London and understand that again and also to get back to being best friends - you empathise with him because you know ultimately he did what he did to save his best friend's life. But John being quite a strait-laced ex-soldier is not too happy.
Do you think there is an evolution in their relationship?
They start from a point where they are almost strangers again, they have to have that moment of trust re-established. It's quite a thing to do to a friend to fake your own death to save a friend, and it takes time.
They rile each other and amuse each other and excite one another and do everything that good friends do but to extremes because of the situation, and the friendship just grows through very heightened circumstances and drama.
It's just a glorious thing to play out, and having such a skilled actor as Martin Freeman to play off - the verisimilitude is bar none and his reaction and comic skill is always exceptional. But the real heart of what he does and why he is so funny is because it's from a place of truth.
In Sherlock you've got this ridiculously theatrical, sometimes manipulative but very real friend which makes for a tempestuous but brilliant odd-couple friendship with all the dynamics of series one and two, just playing out in very different circumstances.
Do you think we will see any changes with Sherlock's relationships with Mycroft in this series?
There's a lovely arc there which gives a lot more insight into them as brothers and where they came from. There is a really interesting glimpse of the hold they have over each other which hasn't really come to light until this series.
It's great, Mycroft becomes the bigger brother in every sense - both intellectually and in terms of power. They are brothers in a family, they are blood relatives and that's explored in more detail which is exciting - funny, but not trite. It's quite profoundly moving I think.
You find out a bit more about both characters, but the most interesting thing is that you find out a lot more about why he is the way he is, you have little hints of back story which just remind people that he grew up - he wasn't just born Sherlock.
Sherlock was somebody who may have been a little bit different as a child but he is very much self-styled; he wasn't born the way he is, there's a lot of conditioning that's made him this genius, thrill-seeking, adventurous, crime-solving hero that he is.
If you could have one of Sherlock's attributes what would it be?
His mental focus, which comes with its faults as well, it's very hard-earned. What I love about him is he is a hero for people who are different and a role model for people who are different, because he acquires a status through his skill that is bar none and I would love to have his mental agility and focus. I just think it is exceptional. And his memory, if I can have two!
It was good - it had a kind of familiarity about it, we had new people joining which was lovely. Given that it was our third time, or fourth including the pilot, it just feels nice and familiar. It's always exciting because the scripts are so good.
Two years have passed since Sherlock faked his death. How do you think this has affected John and how has he moved on?
His best friend has died horribly in front of him, which took a long time to get over, but the way that his life has moved on is that he has fallen in love with Mary. He is leading a functional, normal-ish life really which doesn't have the highs and lows of his life with Sherlock but is certainly a bit more steady. But there is a sadness with John which will always be with him when you lose someone that you love - he is slightly dulled by life.
Could you empathise with John's reaction to Sherlock's reappearance?
I can completely empathise. If someone came into your life after you thought they were dead it would be a major event, and given that Sherlock is the way he is, he winds John up a lot, and that is when they are getting on! But coming back from the dead - John is hit very hard by that.
What do viewers have in store for this series?
Lars Mikkelsen is fantastic, he is a brilliant villain. For those who will miss Moriarty, which is totally understandable, we have another fantastic nemesis for Sherlock and John - he does it superbly. Of course, having Mary Morstan around is good because that adds another dynamic to Sherlock and John's relationship and another dimension to John's life. It was great having Amanda Abbington on board - she was brilliant, I like working with her!
Are you surprised by the massive online reaction to the show?
I don't think any one of us could have dreamt that there would be that sort of reaction. Never mind online, but the entirety of the success that it has had we could never have dreamt about. People used to ask me about The Office. They would say "Did you know it would be this huge thing?" and there was certainly no way we could have known how big Sherlock could have been. I would have settled for it being a really good show that some people really loved. But the fact that it has been feted and honoured - we could never have spotted that coming.
And the online thing is certainly a thread of our fanbase – and it's a very vocal one. I think what's important not to forget is that it's not the only one, there are so many people who just watch it on BBC One when it comes out and love it. But the online thing is surprising in a way because they have almost created another world, especially if you are over 30 because it still seems like that stuff is really new. Especially the sort of reaction we have had, I mean other shows do get it as well, but we are one of those shows that people go absolutely barmy for. And it's nice! I'd much rather that than people ignored it!
What do you think it is that has made Sherlock such a global success?
I guess we were lucky in that we already had Sherlock Holmes, who is one of the most famous fictitious characters in literature. Every incarnation of Sherlock Holmes has been of interest to somebody. And it has been a while since it has been updated. The last time I can think of was Basil Rathbone. So it's a contemporary look at it which hasn't been done for a while, written by three of the best writers around, thought up by two of the best writers around. It's beautifully shot, beautifully designed, rather well acted. It's one of those rare things you can be in where you think every single department is storming it. It looks beautiful, it's written beautifully and it's well cast.
What is it about Sherlock that inspires such loyalty from John do you think?
He's a pretty magnetic bloke. He's very intelligent and there is something mesmeric about his obsessiveness as well. He's the cleverest bloke that John has ever met and he likes that challenge and the share of the danger as well. Because John is ultimately into danger as well, he is a soldier and a doctor so he is around situations that are perilous and a bit tasty, so he responds to that in Sherlock and he just wants to be around him - I sometimes don't even know why!
What's been your favourite episode to date?
I have to say the ones that we have just filmed have been brilliant. Before that, probably Reichenbach was my favourite one to do because when we read it I was really, really excited and I thought the finished product was amazing. It was one of the best things that I've ever done and am likely to do for a while.
But then I found that about the episodes we have just done. It's particularly important you end on a fantastic third one and the third episode in the third series is stellar. Everyone's work on it is great and it's a Steven Moffat special which is always good, but they are all so good! I was very, very excited when I read this one - it's one of the best scripts I've ever read.
The new series will start airing in the USA on Sunday 19th January at 10pm ET as part of PBS's Masterpiece strand.