The show takes a slight stumble in series three, as Sherlock and John are reunited after Holmes' "death" in series two. Moffat's and Gatis's explanation for his fakery is a frustrating cop out, although Sherlock quickly handwaves the situation, as if to say, "the solution isn't necessary," but for a show and character who revolves around The Solution, it may leave viewers somewhat turned off. The other two episodes are fleeting fun and C.A. Magnussen is a slimy new villain, but he doesn't seem nearly as personally interesting as Moriarty. His "threat" is in his influence, although if Holmes hadn't whacked his hive, he wouldn't have had any interest in the sleuth -- Sherlock continues to be his own worst enemy. His solution to Magnussen is... Well, watch it. You're either gonna love it or hate it.
If you liked Series one and two, you'll like three, warts and all. It's still Sherlock; it's still Watson; it's still Moffat and Gatis -- for what that's worth.