5:57 PM PT -- Jasveen Sangha, AKA "The Ketamine Queen," and Dr. Salvador Plasencia appeared in court Thursday and pleaded not guilty.

Sangha was jailed without bond and the judge set Dr. Plasencia's bond at $100,000. Her trial is set for October 8, while his is set for October 15.

4:07 PM PT -- A third person has copped a plea deal in the case ... Dr. Mark Chavez.

According to new legal docs, obtained by TMZ, Chavez pled guilty to conspiracy to distribute ketamine. He was facing up to 10 years in prison.

Chavez admitted to selling ketamine to Dr. Plasencia ... and prosecutors claimed Chavez got the drugs by writing a fraudulent prescription in another patient's name.

10:35 AM PT -- Matthew Perry's assistant, Kenneth Iwamasa, and Erik Fleming, who prosecutors say helped broker the drug deals, already copped plea deals.

According to new legal docs, obtained by TMZ, Iwamasa pled guilty to conspiracy to distribute ketamine resulting in death and serious bodily injury.

Fleming, meanwhile, pled guilty to a pair of charges ... conspiracy to distribute ketamine and distribution of ketamine resulting in death and serious bodily injury.

Iwamasa was facing up to 15 years behind bars and Fleming was staring down up to 25 years in prison ... but in exchange for their plea agreements, prosecutors agreed to seek reduced sentences based on their acknowledgement of responsibility and cooperation with the investigation.

Law enforcement authorities say a broad, underground criminal network was responsible for supplying Matthew Perry with the ketamine that led to his death.

Two medical doctors, Perry's live-in assistant, and someone referred to as the "Ketamine Queen" have been charged with illegally conspiring to provide the drug to the "Friends" actor.

Authorities say the 5 defendants all knew Perry was in the throes of addiction from September, 2023 to the time of his death on October 28, 2023, but cared more about making money off of him than his well-being.

One defendant sent an email asking "I wonder how much this moron will pay [for the ketamine]."

Perry's assistant, Kenneth Iwamasa, allegedly administered more than 20 shots of ketamine in the last 4 days of Perry's life ... this at a time Iwamasa allegedly knew Perry was "spiraling out of control."

Jasveen Sangha, aka "The Ketamine Queen," allegedly sold the fatal dose. Her home was searched and it looked like a drug emporium, with 80 vials of ketamine, thousands of meth pills, coke, Xanax and other drug paraphernalia.

Prosecutors say Erik Fleming was a drug broker who delivered the ketamine from the stash house to Perry's assistant. He threw in ketamine lollipops as an add on for the large purchase.

Authorities say the defendants tried to cover up what they were doing with code, calling ketamine "Dr. Pepper," "cans" and "bots."

After learning of Perry's death, Sangha sent out an alarm to Fleming to "delete all our messages."

TMZ broke the story earlier Thursday ... as law enforcement sources told us multiple arrests had been made, including one doctor.

Multiple law enforcement agencies executed a number of search warrants, where computers, phones and other electronic equipment were seized in order to get to the bottom of who supplied Perry with the ketamine that led to his October death.

The "Friends" star was found unresponsive in his hot tub at his Los Angeles-area home on October 28 ... he was pronounced dead that same day. He was only 54.

It was later revealed Perry had received ketamine infusion therapy for anxiety and depression ... but had last undergone the treatment a week and a half before his death. So, the ketamine found in Perry's system was not doctor-prescribed ... prompting an investigation from both local and federal authorities.