Reference for Eruvin 5:3
וְכֵן שְׁלֹשָׁה כְפָרִים הַמְּשֻׁלָּשִׁין, אִם יֵשׁ בֵּין שְׁנַיִם הַחִיצוֹנִים מֵאָה וְאַרְבָּעִים וְאֶחָד וּשְׁלִישׁ, עָשָׂה אֶמְצָעִי אֶת שְׁלָשְׁתָּן לִהְיוֹתָן כְּאֶחָד:
Likewise, three villages in a triangle — if there are between the two outer ones (and the middle one) one hundred and forty-one and a third cubits, the middle one causes the three of them to be regarded as one. [("in a triangle":) Not necessarily a triangle, but with the third standing afar, opposite the outer ones, in such manner that if it were placed between them there would be no more than one hundred forty-one and a third cubits (seventy and two-thirds cubits for each) between it and each of the outer ones, in which instance the three are regarded as one, so that a man leaving one of them to pass through the others counts the two thousand cubits from the wall of the outer village. This, when there are no more than two thousand cubits between the middle and the outer ones. For since the (men of) the middle one may go to the outer one and vice versa without an eruv, we say that the middle one is regarded as placed between them; but we do not say this when the distance is greater than two thousand cubits.]