The members of the group Mother Nature were arrested after they documented waste run-off into Phnom Penh’s Tonle Sap river.
Cambodia’s law against insulting the king is relatively new and it is not clear how the activists have done so.
The three face between five and 10 years in prison.
The prosecution told the Reuters news agency that “the evidence collected by the police was an insult to the king” although they did not explain exactly how the law had been violated.
Critics have warned that the law – introduced in 2018 – is being used as a tool to silence dissent.
The activists are Sun Ratha, 26, Ly Chandaravuth, 22, and Yim Leanghy, 32, all members of activist group Mother Nature.
They were detained on 16 June while documenting waste discharge into the Tonle Sap river near the royal palace, according to the Cambodian Center for Human Rights.
“The Cambodian government has relentlessly targeted Mother Nature Cambodia,” Naly Pilorge, director of the rights group Licadho, told Reuters.
“This marks an escalation with the outrageous charges of ‘plotting’.”