Award-winning filmmaker Kalyanee Mam submitted an essay to VOA Khmer following the murder of journalist Taing Try.
The three men are also accused of possession of illegal weapons.
Cambodian and UN officials say they still need more funding to continue the work of the Khmer Rouge tribunal, despite waning interest from some donors.
More than 1,000 people marched on the National Assembly on Monday, many of them displaced from land disputes, to mark World Habitat Day.
Thach Sitha, president of the Association of Kampuchea Krom, which advocates for the Khmer ethnic minority living in Vietnam, told protesters Wednesday he has received death threats in recent days by phone.
Some 200 protesters have gathered in front of the embassy over the past five days to demand an apology from an official there for comments he made about Kampuchea Krom.
Vietnamese Embassy officials declined to comment on the protest, which followed a two-day conference in Virginia to discuss issues faced by Cambodians, Hmong and Montagnards.
Cambodians and Laotians gathered in front of the White House in Washington on Monday, claiming that Vietnamese policies in the region are hurting their respective countries.
Cambodia has nearly 3 million Internet users, most of whom go online via their mobile phones.
Hor Namhong also defended, more broadly, Cambodia’s rights efforts, pointing to the high number of NGOs in the country, as well as media that criticize the government.
The decision was made following a meeting Wednesday morning between city officials and community organizers.