The victim, a 32-year-old from Cambodia's ethnic minority Bunong community, was kidnapped from her Kratie province home on October 22.
The perpetrators took her about 2 kilometres away and broke her neck before dumping her at the side of a road, The Cambodia Daily reported.
Two men and a woman were arrested, and a fourth man was being sought by police.
The suspects confessed to the killing, the report quoted a local police chief, Chhin Sambo, as saying.
They said they acted following a number of deaths and illnesses in their community, which they blamed on the victim.
There have been at least five cases of superstitious villagers committing such vigilante killings so far this year.
In one, a traditional healer was set upon by an angry mob and bludgeoned to death; another case saw a so-called sorcerer decapitated.