If there were two, some of the cubits of one overlapping the cubits of the other, they may bring [their food] and eat in the middle, provided that one does not carry from his into his friend’s. If there were three and the middle one’s [area] was overlapped by the other two, he is permitted with them, and they are permitted with him; but the outer two are forbidden with each other. Said R’ Shimon: To what is this comparable? To three courtyards which open one into the other and which open into the public domain. If the two made an eruv with the middle one, it is permitted to them, and they are permitted to it; but the two outer ones are forbidden one to another.