The confession by the 38-year-old woman, named as Jolanta K, partly clears up the discovery in August of the remains of five infants at her family house in Czerniejow, near the eastern city of Lublin.
Autopsies and DNA tests found the five victims to be the children of Jolanta K and her husband Andrzej, and that they were killed between 1992 and 2002, prosecutors said on Tuesday.
"The suspect...admitted to deliberately killing three of her children - two new-born boys and a girl," prosecutor Andrzej Lepieszko told a news conference in Lublin.
The pair, who face possible life sentences if found guilty of murder, were detained at the weekend in a police hunt which followed the discovery of the infants' remains on 20 August.
Two of the couple's four living children made the gruesome discovery when they took the barrel out to a field to empty it after their grandmother complained of an unpleasant smell.