With this the total arrested included four - all of them police officials attached to the Nedumkandam police station.
The Crime Branch had earlier arrested Sub Inspector K.A. Sabu and senior civil police officer Sajeev Antony and charged them with murder.
Rajkumar, 49, a small-time financial agent, allegedly died in custody on June 21 after being subjected to brutal third degree torture at the Nedumkandam police station.
The latest arrests come at a time when the Judicial Commission of Justice K. Narayana Kurup (retd), is all set to commence its job.
This is the fifth allegation of custodial death against the Kerala Police since 2016, when the Left Democratic Front (LDF) government came to power.
According to reports, Rajkumar was rounded up on June 12 by a group of people who he owed money to, and handed over to the Nedumkandam police. The police however, recorded his arrest on June 16 and later shifted him to the state-run Peerumedu Taluk Hospital, where he died on June 21.
Though the post-mortem mentioned pneumonia as the cause of death, the autopsy report revealed 22 injury marks on Rajkumar's body, most below the thighs, indicating that he was subjected to third degree torture while in custody.
Idukki's Superintendent of Police K.B. Venugopal has already been transferred.
According to sources in the Crime Branch, which is probing the case, more arrests of police officials cannot be ruled out.