The protagonist is perhaps a doctor. Or maybe a lawyer, teacher... But most probably a medic, since his services were needed immediately.
So he, an outsider, was called into some remote town to treat a little girl. Insiders were not happy but saw it as required risk.
Alternatively, perhaps a girl was brought to him, and after treating her few days later he wants to check if her health is going better and visits her hometown.
The guy starts noting the weird girl's behaviour, like she had memory gaps or split personality. He thinks, looking at items around, there are three girls with radically different interests and tastes live in the house, but sees there is a single child's bed there. Step by step, he deduces that EVERYONE in the town has three personalities co-inhabiting their bodies. They stick together in this remote places because they don't want to be seen as a potentially dangerous "alien race" of sorts.
In the end he moves away from the town as that girl's elder sister, who studied faraway to become a town medic, comes back from a city.