Reuters reports that the suspected attacker is a 21-year-old female student. The attack reportedly took place at an adult education school in Warsaw.
According to the Polish news agency PAP, the victims are two 17-year-old female students and a 24-year-old woman. They were attacked in the school's administrative office while submitting documents. Police say there was no apparent connection between the suspect and the victims.
This is the third knife attack of its kind in Poland this week.
On Saturday morning, police announced that a total of 76 people had been arrested on Friday and around 50,000 school buildings had been inspected nationwide as authorities investigated whether the incidents could be linked to a social media challenge.
Among those arrested were many minors who had made online threats to carry out similar attacks.
On Thursday, a 37-year-old caretaker was killed and two teachers were injured in a knife attack carried out by a 15-year-old student.
The previous day, a 19-year-old student attacked fellow students and staff at a vocational school, injuring nine people, two of them seriously.
Prime Minister Donald Tusk announced increased police checks at schools and suggested that social media platforms and video games were partly responsible for the attacks.
However, it remains unclear whether the incidents are linked to a social media challenge, as initially suspected.
