Stephen Collins -- Cops Looking to Find 3rd Sexual Abuse Victim

The mystery girl who Stephen Collins admitted he sexually abused was a babysitter for the actor's daughter ... and we know cops are on the hunt for her.

TMZ broke the story ... Collins confessed on audio that he molested or exposed himself to 3 girls. Two of them have come forward ... one has gone to the NYPD and another to the L.A. County Sheriff. NYPD detectives know the 3rd victim was sexually abused in NYC and they now know who she is and are trying to track her down.

TMZ has learned ... the woman was around 13 when she babysat Collins' daughter 18 years ago. Collins' wife, Faye Grant, told cops Collins admitted to her he would repeatedly drive with the babysitter in the front seat with their 6-year-old daughter in the back seat and expose his penis to the babysitter.

Grant says Collins tried making excuses, saying the babysitter didn't see his penis when he exposed it and his daughter didn't see it either.

Cops want to talk to the woman. So far, they haven't made contact.