Right, first of all here's a panel that Steven Moffat, Mark Gatiss and Sue Vertue did for Nerd HQ after the official Sherlock panel. There are some references to ACD canon characters/events as relevant for S3, but no real spoilers otherwise:
Click to view
Also, the titles of the three episodes
have been confirmed:
S3E1: The Empty Hearse
Written by Mark Gatiss, Directed by Jeremy Lovering
S3E2: The Sign of Three
Written by Steve Thompson, Directed by Colm McCarthy
S3E3: His Last Vow
Written by Steven Moffat, Directed by Nick Hurran
Then here's something from the official Sherlock panel that might disappear again - they showed a clip of S3E2! No video, but someone
uploaded an audio recording of it to Youtube, and
here is a transcript. For those spoiler-free who know which ACD story the S3E2 title was inspired by, you know about the new character and the significant event taking place in this clip, so you should be safe listening/reading. The scene is mostly John and Sherlock and just from the audio I can say OMGOMGOMG IT IS SO GOOD AND FUNNY AND AWWW AND I CANNOT WAIT TO SEE THIS SCENE!!!!
When is it S3 airtime yet?!