Chicago police said three adults were found unresponsive at the Edgewater neighborhood's Judge Fisher Apartments at about 6:30 a.m.
The high-rise at 5821 North Broadway is a Chicago Housing Authority property for seniors.
The Chicago Fire Department said the incident was called in as a hazardous materials situation. When they got to the building, they found no hazardous readings, and there was no fire.
CFD said a man was pronounced dead at the scene, and two women died at Weiss Memorial Hospital. What caused the deaths was not immediately clear.
The Cook County Medial Examiner identifies the victims as Linday Daly, 70, of the same residence, David L. Edwards, 53, of 4600 blk. of N. Kimball and Royce Daugherty, 38, of the 1200 blk. of W. Farwell,
Police said Area Three detectives are conducting a death investigation, but did not immediately provide further information.