Mishnayos Eruvin Perek 5 Mishnah 3

וכן שלשה כפרים משולשים – on actually a triangle, but the third stands from afar corresponding to between the outer ones (i.e., towns) , and everything, for whereas, the middle [town] enters between them and there isn’t anything between this one and that one other than one-hundred and forty one and one-third cubits , which are seventy cubits and two-thirds [of a cubit] to this one, and seventy and two-thirds [of a cubit] to that one, and similarly, to the side of the other outer city/town. There is nothing between the middle one (i.e., town/city) and it other than one-hundred and forty-one cubits and one-third [of a cubit], all three of them (i.e., the towns/cities) are thought of as one [city/town]. But the person who leaves from one of them to to go through its neighbor counts from the wall of its outer neighbor. And how much should there be between the middle [city/town] to the outer [city/town]? Two-thousand cubits, for since the middle [town/city] is capable of going to the outer one and outer one to the middle one without an Eruv, we state that we see it as if the middle one is placed between them. But if it is further than two-thousand [cubits], we don’t say that we see it.