BEIJING — Three Tibetan monks in central China set themselves on fire this weekend, raising to 15 the number of suicides in the last year by Buddhist clergy members protesting aspects of Beijing’s rule in Tibet.

The deaths suggest that self-immolation is gaining favor as a form of political protest for Tibetan clergy. And they underscore the challenges the Chinese authorities face in controlling more than five million ethnic Tibetans living in what China calls the Tibet autonomous region and adjacent Sichuan and Qinghai Provinces.

China’s central government has cracked down hard on religious activism in Tibet since ethnic riots in 2008 killed 19 people, many of them Han Chinese migrants, severely embarrassing rulers in the months leading to the Summer Olympics in Beijing. Human-rights activists say that hundreds of Tibetans were arrested afterward, and that some of them died in custody.

Western journalists are largely barred from traveling to the region. But reports by others indicate that monasteries and other gathering places have been placed under strict surveillance.