The rescued children were an 8-year-old girl, 5-year-old boy and 4-year-old boy, Clark said at a press conference Thursday afternoon.
Cummings had a lengthy discussion with Malone's distraught husband, who assured him that his wife was "going to be OK". One body has been found at the scene.
The fire happened in the 4200 block of Springwood Avenue. The cause of the blaze was still under investigation.
Maryland Representative Elijah Cummings identified the mother as Katie Malone, a special assistant who had worked in the congressman's office for almost 11 years, according to a press release from his office.
Baltimore City Fire Department spokesman Roman Clark said firefighters arrived to see large flames coming from all three floors. He says she serves constituents in the areas of immigration, postal services, and the military, including service academy nominations.
"My staff is a family and this unimaginable tragedy is shocking and heartbreaking to us all", Cummings said in a statement. Crews were awaiting the arrival of heavy-duty equipment that would help them remove debris from the dwelling, he said.
By 9 a.m., the six children who remained unaccounted for were believed to have died, according to WJZ.
Mrs. Malone was also hospitalized but is expected to survive.
Mr. Malone said he is "still in shock" about the fire.
The third floor collapsed and the second floor partially collapsed. The blaze is contained, but hasn't been brought under control.