The 20-year-old boyfriend of the triplets' mother was watching the girls Sunday night while she was out playing bingo, police spokeswoman Diane Richard said Monday.
The boyfriend told police that he noticed the girls following him up the 14-step staircase and he told them to go back down the steps of the Pittsburgh home.
The triplets were wearing high heels when the toddler nearest the top of the stairs fell, knocking the second child into the third, the boyfriend told police. The boyfriend told police that one of the toddlers fell to the landing below, and she was the most seriously injured, Richard said.
When police arrived at the house at about 8:15 p.m. Sunday, one of the triplets was not breathing. All three were taken to the Childrens Hospital of Pittsburgh where two were treated for bruises and a cut lip and released after an overnight evaluation. The third was listed Monday in "very critical condition with head trauma," Richard said.
The Sex Assault and Family Crisis unit of the Pittsburgh police is investigating. No charges have been filed against the boyfriend or the triplets' 25-year-old mother, Richard said.