Police have arrested three teenagers for allegedly assaulting a 17-year-old girl in a toilet in Mong Kok last month, and said the case had to do with money and relationship issues.

The three girls, aged 13 to 16, were picked up on suspicion of assault occasioning actual bodily harm.

Officers said the 17-year-old filed a police report on Friday, saying she was attacked by several people in a bathroom at a shopping mall in Mong Kok on October 17.

"Our investigation showed that the alleged victim and the suspects knew each other," acting chief inspector Wong Wing-hang said in a media briefing.

"The suspects [allegedly] assaulted the victim, during which someone filmed the incident and posted it on social media."

Police said she was slapped, punched and kicked.

Officers added that the girl suffered injuries to her face and mouth, as well as a headache, but did not seek medical attention.