They said they belonged to a group called the Masonic Fraternal Police Department, which they claimed dated back to the Knights Templar.
One of the trio is a junior aide to California's attorney general.
Uniforms, weapons and vehicles were found in premises linked to the group.
Brandon Kiel - an aide to state Attorney General Kamala Harris - David Henry and Tonette Hayes were arrested on 30 April and released later that day.
"When asked what is the difference between the Masonic Fraternal Police Department and other Police Departments the answer is simple for us. We were here first!" the group's website page reads. "We are born into this Organization, our bloodlines go deeper than an application."
According their website, the Masonic Fraternal Police Department was created by the Knights Templar in 1100 BC - although the crusader order was set up more than 2,000 years later.
Authorities began investigating the group after police chiefs in southern California received a letter in late January that announced Mr Kiel would be serving as "chief deputy director" of the group.
The letter claimed there were 5,686 lodges and that the department "will be able to acquire intel that is not accessible to non-fraternal entities".
Los Angeles County sheriff's spokeswoman Nicole Nishida told the Associated Press news agency that there were no indications that the group had carried out any law enforcement activities.
Three people have been charged in California with impersonating law enforcement officers after claiming to operate a police department with jurisdiction in 33 states.