Avon and Somerset Police made the tragic discovery after officers attended a concern for welfare call in Blaise Walk, Sea Mills, at around 12.40pm.
Neighbourhood Chief Inspector Vicks Hayward-Melen said: “This is an incredibly tragic and heartbreaking incident in which three children have sadly died.
“My sincere condolences go to the children’s loved ones and we will be ensuring they are offered support through our family liaison unit.
“We believe this to be an isolated incident and there is no further risk to the wider community, however officers will remain at the scene to provide reassurance to anyone who has any questions or concerns.
“Over the coming days, residents can expect to see an increased police presence and we thank them for their cooperation during this time.”
As is usual in these circumstances, we have notified the Independent Office for Police Conduct (IOPC), due to prior police contact.