In The Sign of Three, whose wedding is Sherlock attending?

In “The Sign of Three,” Sherlock is attending the wedding of John Watson and Mary Morstan—John and Mary. The episode is built around their wedding day and reception, where Sherlock serves as John’s best man.

Sherlock struggles to write and deliver his speech, then turns it into a rambling account of cases involving a seemingly murdered soldier, the Mayfly Man, and a threat against Major James Sholto. His investigations eventually reveal that the wedding photographer is an attempted murderer targeting Sholto.

The episode’s title echoes Arthur Conan Doyle’s novel “The Sign of the Four,” but the television story focuses heavily on friendship, marriage, and changing relationships. Sherlock realises that John’s marriage will alter their partnership, leaving the reception alone after the celebrations.

A final deduction gives the episode its title meaning: Sherlock notices signs that Mary is pregnant. Thus the “three” refers to John, Mary, and their unborn child, not to another wedding pair.