Ah, the triple-decker (or three-decker, if you’re more old school). What makes it special is that it is the vernacular style of Boston but is something that largely goes unseen in the common “Boston” tourist experience. Below is map on the diffusion of three-decker styles into Dorchester from Roxbury and South Boston, from The Three-Deckers of Dorchester (PDF) by Arthur J. Krim, hosted at the Sidewalk Memories site. The paper notes that fire laws in the central city—which, wouldn’t you know it, more or less coincides with tourist Boston—forbade wood construction, hence the triple-deckers kind of being the locals’ special secret today.
I’m not sure if these two styles are so identifiable today, as the author noted that they merged eventually, but one can certainly find a lot of both flat roofs and pitched roofs. It would be interesting to see what the geographical distribution of each is now.