A man, woman, and a 5-year-old boy were found stabbed to death inside a Bronx apartment building early Sunday, police and sources said.

Police responding to a 911 call at 674 East 136th Street in Mott Haven first discovered the man, believed to be in his 20s, with stab wounds to the chest in the hallway of the first floor of the apartment building, at around 6:40 a.m., cops and law enforcement sources said.

As they investigated, the officers looked into the window of the nearby apartment unit — and broke inside after spotting the child, according to the sources.

The woman, whose age wasn’t immediately known, was also discovered inside the apartment, with fatal stab wounds.

Neighbors said all three had recently moved into the unit, according to sources.

They were pronounced dead at the scene by EMS, police said.

An autopsy was being conducted to determine the cause of death.

Their names were not immediately made public as police worked to identify and notify any relatives.

The police investigation was ongoing.