Unidentified gunmen opened fire as they sought to remove road blocks set up by people protesting against Ulises Ruiz, the city's state governor, on Friday.
Bradley Will, a cameraman working with Indymedia New York, was shot in the chest and died before reaching hospital, the news group said on its web site.
"He told me it was getting sketchy," Dawn Neary, a friend of Will, said. "He would always put himself on the front lines. He was a courageous guy. He really believed in truth, public awareness and justice."
Oaxaca's state prosecutor's office said two others, including a protesting teacher, were shot dead in separate attacks.
Nine people have been killed in the conflict that began five months ago when striking teachers and demonstrators occupied much of the state capital.
Lizbeth Cana, Oaxaca’s attorney general, blamed the violence on the protesters, who she likened to an urban guerrilla group. She said the armed men were angry residents defending themselves.
Several people were injured in the
"The people are fed up with permanent violence, threats and kidnappings," Cana said.
However, protesters accuse the governor of sending the armed men against them.
"Ulises Ruiz is trying to massacre our people," said protester Antonio Garcia.
Protesters took over areas of the historic city five months ago, building barricades, driving out police and burning buses.
They accuse Ruiz of rigging the 2004 election to win office and using violence against his opponents.
Friday's clash came a day after teachers agreed to end their strike in a move that was expected to calm the protests.
The teachers have been camped out in Oaxaca city's colonial centre since May when they first walked out to demand higher pay and better working conditions.
After clashes between police and demonstrators in June, they extended their demands to include a call for the resignation of Ruiz and have been joined by activists, students and Indian groups.