Phuon Keoreaksmey, Long Kunthea and Thun Ratha were handed sentences of between 18 and 20 months for "incitement to disturb social security," reported local outlet Voice of Democracy.
The trio, who were also fined 1,000 dollars each, are members of the environmental activism group Mother Nature.
The court also convicted in absentia the group’s co-founder Alejandro Gonzalez-Davidson and another member Chea Kunthin.
The three activists were detained in September. They were planning a one-person march to raise awareness about the infilling of a Phnom Penh lake.
Mother Nature has been vocal against environmental crime in Cambodia, taking aim at issues such as sand dredging, logging and the impact of hydro power.
In a statement, Human Rights Watch deputy Asia director Phil Robertson slammed the court’s "outrageous and unacceptable" persecution of the activists.
"These environmental activists show incredible bravery by shining a light on corruption and rights abuses connected to crony business projects that threaten Cambodia’s natural resources and biodiversity," he said.
"Today’s convictions are part of the Cambodian government’s continued vendetta against Mother Nature, a thorn in the government’s side that the officials are now moving to destroy through bogus criminal charges." - dpa