Linda Traitz told CNN that she and fellow accuser Therese Serignese are joining the suit, which was originally filed by Tamara Green.
Traitz, Serignese and Green are three of at least 23 women who have accused the comedian of sexual misconduct ranging from groping to rape. And after each came forward with the allegations, they were called liars by representatives for Cosby, the defamation suits says.
Cosby's attorney, Martin Singer, has called the spate of sexual assault accusations against the comedian "ridiculous."
He told CNN it defies common sense that "so many people would have said nothing, done nothing, and made no reports to law enforcement or asserted civil claims if they thought they had been assaulted over a span of so many years."
CNN was unable to reach Singer for comment about the latest development.
Here are the claims of the three women in the suit:
Linda Joy Traitz
Traitz said Cosby pushed himself on her when she worked as a waitress at a restaurant in Los Angeles that Cosby co-owned when she met him in 1969.
Traitz was either 18 or 19 at the time, she said; she did not recall exactly. But she was fresh out of high school and new to the city, she said.
Cosby offered her a ride home but on the way, told her he'd like to swing by the beach with her in his Rolls Royce, she said. They parked, and he offered her drugs, an array of brightly colored pills, "to relax," she alleged.
She said no, a few times. "He kept offering me the pills," she alleged. She claimed that he then groped her chest, pushing her down in the seat and toward the door, and tried to lie on top of her.
Singer, Cosby's attorney, has said her account was not true.
"Ms. Traitz is the latest example of people coming out of the woodwork with fabricated or unsubstantiated stories about my client," Singer has said.
Numerous arrests speckle Traitz's rap sheet, including on suspicion of theft, battery and types of deception -- fraud and impersonation -- often in connection with drug possession.
But Traitz spoke openly about her record to CNN, including her last arrest in 2008, which led to more than three years in prison for drug trafficking and possession.
"When I got sent to prison ... it saved my life," Traitz said. "It was a great experience for me." She said it ended her addiction to pain pills, and she's been clean ever since.
Florida nurse Therese Serignese, 57, told ABC's 20/20 that she was a 19-year-old model visiting Las Vegas when Cosby told her to take some pills in a private dressing room following a performance.
After taking the pills, she remembered "feeling drugged, and I was kind of leaning forward, and he was behind me having sex with me. And I -- I remember it because it was not good."
Serignese never made public accusations in the immediate aftermath. She explained her decision in an article she wrote in Friday's New York Daily News.
"Cosby was everywhere. Everyone thought he was a great family man. I knew he wasn't. I just couldn't prove it with anything but my word. There was no video camera or DNA evidence. No one else had accused him publicly yet," Serignese wrote.
Serignese says she wanted to testify in support of Andrea Constand, who filed a civil lawsuit against Cosby.
"When the first victim came forward in 2005, I was angry that he called her a liar. I wanted to back her up. But even then I worried about the repercussions. I had younger children then," Serignese wrote.
"Ten years ago, the climate still wasn't right," she said. "We would have all been humiliated. Now, I could see it was time, and I would be safer. I wouldn't be alone. It took me 38 years to feel safe.
According to her lawsuit, Green was an aspiring model in 1970 who met Cosby through a mutual friend.
The two met for lunch, even though Green was feeling ill, the document states. Cosby allegedly gave Green some pills, saying they were cold medicine -- but Green started feeling "weak, dizzy and woozy."
Cosby took her to her apartment and started "groping me and kissing me and touching me and handling me and you know, taking off my clothes," Green told NBC's "Today" show.
Cosby allegedly left two $100 bills on her coffee table and left, the lawsuit says.