Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 379…
“ Paragraph 1 - If there are two yards with three houses in between them, each open to each other, and they want to make an eruv together, one yard goes through the house adjacent to it, and other yard does the same. All three houses will be permitted to the yards, as every adjacent house is considered a gatehouse and need not give bread, and the middle one is the house in which the eruv is placed and does not need to give bread. ”
