Eruvin 57: Three Cities and the Precise Distance between Them

A new mishnah - on adding a karpef (the extra space of 70 amot), but when? Depending on the space between two cities, or between three, for that matter... Now, about these three villages, the ability to go far is apparent, but why they needed to is not. The three villages don't have to be in a line! But were you to slide them into position of a line, then you'd have this distance between them (the amount of techum plus two karpefs).