There were so many things to like about The Sign of the Three, the last episode of BBC Sherlock's third season. There was enough drama to (hopefully) give us plenty to chew on while we wait for the next one. As usual, the costume design was fantastic. Mary's style in particular was inspired by a vintage 1920s/1930s aesthetic. Even though the wedding itself went a bit pear-shaped thanks to crimimal conspiracy and murder, the wedding photos probably came out looking fantastic.
Yeah, that's adorable. The dress was designed by Jane Bourvis, a designer known for gorgous hand-made antique lace dresses, whose work is often seen when British royalty are walking down the aisle. The silhouette has a classic look, amplified by the lace details. It's an interesting choice, especially since the Watsons are such an extraordinary couple. Perhaps thr design choice was a way for the show to refer back to the original stories, by evoking the era in which they were written. If so, they were a few decades off, but it's a nice touch all the same. It also fits in to Mary's personal style: while she does generally wear modern casual outfits, her 'dressed up' style is very 1920s.
Mary's make up matches the gown. It's very simple and understated, which closely mirrors the aesthetic of the period that inspired the look. It's definitely modern, but classically inspired. The white flowers have some nice symbolism as well- white roses and lillies. While this used to symbolize chastity, I think now they can be interpreted as a new beginning. Mary is starting fresh with this marriage, embracing her new, more peaceful life. While dresses like these are pretty much one-of-a-kind, vintage wedding dresses often end up in thrift shops for well below what they were originally worth. You'd have to get lucky, but keep an eye out for one, especially in early springtime (when most people start cleaning out their attics).
Her engagement ring also has a very classical look. When John presents it to her, it looks a bit tarnished, which makes it seem like it was a family heirloom. There are similar rings on the market today, though thrifty cosplayers might have some trouble finding costume versions. Again, a thrift shop of flea market might be your best bet.
The pearl bracelet is much easier to duplicate; stores like Forever 21 (above), Express, and H&M carry similar pieces for really cheap. That, plus a clear gloss manicure add an understated elegance to the look.