A beer drunk in 18th century London made up of three types of beer, commonly pale ale, mild (fresh brown beer), and stale (matured brown beer).
While there is a lot of lore surrounding three threads, there is not much to back up some of the claims around it, such as it being the precursor to porter. Porter existed contemporaneously with three threads and did not grow out of it.
Another claim made about three threads is that it was drawn by the publican from three separate casks. However, it has been hypothesized that the beer was delivered to pubs already mixed in one cask.