The dictionary entry for several lists the following as one of the definitions:
Single; distinct: "Pshaw! said I, with an air of carelessness, three several times" (Laurence Sterne).
So in this case several doesn't mean a relative number (like a few), but instead that there were three distinct fires.
This unusual definition is related to the etymology of "several":
early 15c., "existing apart," from Anglo-Fr. several, from M.Fr. seperalis "separate," from L. separe (ablative of *separ "distinct"), back formation from separare "to separate" (see separate). Meaning "various, diverse, different" is attested from c.1500; that of "more than one" is from 1530s, originally in legal use.