|The third stone|
There's a lovely tale associated with them. Apparently Old Nick (you know, the fallen angel) fell out with the neighbouring manor of Aldborough for some reason and decided to hurl huge stones at the residents as a punishment. He stood on How Hill, near Fountains Abbey, to get a better reach, but his throw wasn't up to it and the stones fell short.
Current archaeological thinking is that they were probably a single row. According to 16th century reports there were once at least four stones 'set in a straight and direct line'. (William Camden, 1560). The fourth stone is believed to have been incorporated into a bridge in the grounds of Aldborough Manor.
For more information see the Boroughbridge website.