While the narration is in the third person, it's clear that it's not a single individual. The narrator uses plural pronouns ("we," "our") and refers to collective knowledge and memories ("we knew that ...," "the whole town had ...").
The use of this collective voice allows Faulkner to present a more multifaceted and nuanced perspective on Emily Grierson and her life in the town of Jefferson. It allows the reader to understand how the townspeople viewed Emily, their changing attitudes towards her, and the way their collective memory shaped the story of her life.