The children were discovered Thursday morning on the 1300 block of Niles Street with very little food and in need of medical care.
The on-site apartment manager said she spotted the little children at the open gate facing Niles.
"So, my concern was to go down and then look at their safety," she told Eyewitness News. The manager did not want to be identified.
She said the kids' apartment door was open, so she called in to find their parents.
"I continued on for, I don't know, a couple seconds, and then I realized, OK, nobody's here," the manager said.
At that point, she contacted another neighbor, who knows the kids' family. She said he was also didn't find anyone in the apartment, and he called police.
The manager said she had started to take care of the children by the time the officers got there. She'd begun by checking the diaper on the baby.
"It was totally, totally soiled. I've never seen a diaper like that," the manager said. And, she immediately reported that to an officer. "I go, 'This child has got to go to the hospital, he's infected really bad.'"
Police tell Eyewitness News one child was taken to the hospital by ambulance, and officers took the other two there.
They are all now in protective custody.
Police said the mother left the children in the care of a relative two days ago, and the relative may have abandoned the children.
Eyewitness News discovered there actually were two people inside the apartment. Eddie Gonsalez said he was sleeping, and police woke him up.
"The uncle was supposed to take care of his nephews, and he didn't do the job," Gonsalez said. "He just left, and so (the officers) took the kids."
The teenager said the kids' mother went to the hospital because of a kidney stone.
"It's just me and my girlfriend," he said.
Police told Eyewitness News the teens found in the apartment were 15 and 14 years old. Gonsalez said he had fed the toddlers on Wednesday night, but said there was no food in the apartment Thursday morning.
Code enforcement officers told Eyewitness News the unit was posted as a danger to live in because there were no working utilities.
The property manager said the unit had originally been rented to a couple with the three small children, and the tenants seemed like good ones.
But, the property manager said the man had moved out, and after that there had been parties and problems in the unit when the mother continued living there.
"We gave her a 30-day notice already a while back, and we called the gang unit because some gang activity was going on," the property manager said.
He did not want to be identified, saying he's been getting threats.
The manager said he did get a look in the apartment.
"It's just a mess, a lot of mess," he said. "Very little furniture, there's a lot of trash in there."
The teen who had been staying there said the mess was caused by the three kids, and they tried to clean up.
"I think it's sad, they should never have taken the kids away," Gonsalez said. "I just don't think it's right what they did."
The apartment manager insists the children were not being cared for. She said officers told her when they found the teenagers in a second room, they could hardly wake them up.
"We were right in the same room they were laying down there, and we were trying to wake them," the manager said the officers told her.
She's shocked: "And (the teens) were supposed to be taking care of the children."
Police say it appears the children are between 1 and 3 years old.
No arrests were announced. Anyone who can provide useful information for the investigation is asked to call police at (661) 327-7111.