The first episode of "Sherlock" Season 3, "The Empty Hearse," introduced Watson's lady love, Mary. At the end of the episode, it was revealed that Watson finally managed to propose to Mary and they are engaged. Despite Sherlock interrupting her proposal scene, Mary liked Watson's best friend. She and Sherlock also teamed-up and took a wild motorbike ride to save Watson from getting burned alive.
Watson and Mary are going to get married in the second episode, "The Sign of Three." Since Sherlock is alive, one cannot imagine someone else to be Watson's best man. At the wedding ceremony, Sherlock will be facing one of the biggest challenges of his life. He has to give a best man speech on the big day. In the preview of the "The Sign of Three," Lestrade wonders what is the worst that could happen if Sherlock is the best-man, while Mycroft says "don't lose it."
The preview of the upcoming episode suggests that Sherlock is a mess when it comes to being the best man. It is a completely new domestic situation for him. At the wedding, the happy moments have the company of a mortal danger. Sherlock can be seen telling in the video, "lets play murder."
"There is man in there about to die. The game is on. Solve it," Watson can be seen telling Sherlock in the preview video. The synopsis of the episode indicates that bridesmaids will help Sherlock solve the case and stop the murder. It will be interesting to see what Mary has to say as her wedding isn't going to be a normal one.
BBC's Official Synopsis of "Sherlock" Season 3, Episode 2 "The Sign of Three": "Sherlock faces his biggest challenge of all - delivering a Best Man's speech on John's wedding day! But all isn't quite as it seems. Mortal danger stalks the reception - and someone might not make it to the happy couple's first dance. Sherlock must thank the bridesmaids, solve the case and stop a killer!"