BANGKOK — After spending more than a decade and nearly $300 million, the United Nations-backed tribunal prosecuting the crimes of the Khmer Rouge has convicted just three men.

It appears now that they could be the only people to answer in court for the deaths of at least 1.7 million Cambodians from 1975 to 1979 in one of the worst episodes of mass killing in the last century.

Three more potential defendants have been investigated by the tribunal, an ungainly mix of Cambodian and international prosecutors and judges. But because of resistance on the Cambodian side, there are serious doubts that their indictments will proceed.

“From the start, the Cambodian government had a very different idea about how many people would be tried, and their view appears to be prevailing,” said Alexander Hinton, an anthropology professor at Rutgers University-Newark. “The court’s legacy will be tainted and greatly diminished if it fails to try further cases.”