Commissioner Edward Davis says police responded to the city's Mattapan neighborhood just after 1 a.m. on Tuesday.
Two men and one woman found in the street were pronounced dead at the scene. Department spokeswoman Elaine Driscoll says the toddler, aged either 2 or 3, died at the hospital. She says a man in his 30s taken to a hospital is not expected to survive.
No names were released and there were no arrests.
Davis says investigators think the victims knew the shooter and is appealing to the public for help in apprehending the suspect.
Driscoll says police are looking for a silver or gray Ford Explorer that may be connected to the shooting.