one's next three neighbors; the three nearest houses
Refers to the three houses on either side of one's own home, often used in the context of neighborhood mutual assistance or community ties.
We have long-standing relationships with our three nearest neighbors on both sides.
Compound of 向こう (mukō, 'opposite, across') and 三軒 (sangen, 'three houses'). The exact historical derivation is uncertain, but the phrase conventionally refers to the three houses opposite or adjacent to one's own.