A US judge has refused to throw out a libel suit filed by three women against Bill Cosby the same day the comedian was set to testify in a separate molestation case.

In a 38-page ruling a judge in Springfield, Massachusetts, denied Cosby's motion to dismiss the suit filed by Theresa Serignese, Tamara Green and Linda Traitz.

The three women say Cosby sexually assaulted them years ago, then called them liars when they went public with their accusations last year.

The decision is the latest legal setback for Cosby, 78, coming on the same day he was called to testify under oath in a Los Angeles lawsuit accusing him of drugging and sexually molesting a 15-year-old girl.