The tragedy unfolded in a remote area of Kratie province on Saturday when Say Phalla, 25, climbed down the 10-metre-deep well after his pet plunged into the water.
"Say Phalla used a rope to climb down the well to save his dog but suffocated due to a lack of oxygen at the bottom, disappearing in the water," local police chief Men Bunthoeun told AFP by telephone.
A male neighbour, followed by Phalla's 64-year-old mother, then climbed down in a desperate attempt to save him and the pet, but they also passed out.
It was unclear how the dog, which also died, crossed the barrier encircling the well.
Bunthoeun said authorities retrieved the dead bodies after pumping water out from the well upon being alerted to the incident by other neighbours.
Last month seven people including four children died in a well in north Cambodia's Siem Reap province after trying to rescue an 11-year-old boy who had climbed in to retrieve 3,000 riel (25 baht).
Cambodia is one of the poorest countries in Southeast Asia, with around 20% of the population — or 2.8 million people — living in poverty, according to the World Bank.