The "Menard triplets" are three 19th century flounder houses in Soulard located on the west side of Menard Street just south of Russell. Many flounder roofs simply form a half-gable, running down from one side of the building to the other. These houses have a hip to their roofs that allows for a front-facing dormer. Still, the roof form is within the flounder house range. St. Louis seems to have the largest concentration of flounder houses, which are found in few American cities (Alexandria, Virginia and Philadelphia have them).
The center house (left here) was extended to the south to meet the northern house, creating a "mousehole" entrance to the gangway.
A plaque on the wall of the center house tells some of the story of the houses, including a wide range of salvage pieces that went into rehabilitation of the center house. Plaques like these are a great part of the urban fabric in that they allow buildings to tell some of their own story. Forget the Internet or a guidebook -- the best way to explore is on foot, and the best way to learn about historic architecture is to study the buildings themselves. A few more clues always help.