According to a statement posted on the local government's website, the attack occurred Tuesday evening in Pishan county in southern Xinjiang, home to China's Uighur ethnic minority. It said the three also injured five others before being shot dead by police.
Uighurs are predominantly Muslim Turkic-speaking people distinct from the Chinese-speaking Han national majority. They have long chafed under the rule of Beijing, more than 1,800 miles (3,000 km) away from the provincial capital of Urumqi.
While Xinjiang is one of China's five autonomous regions, its Uighur residents are often prevented from leaving the region and face other restrictions enacted by Beijing, which has ratcheted up security dramatically since deadly anti-government riots broke out in Urumqi in 2009.
The statement did not say whether Tuesday's assailants were linked to ETIM (East Turkestan Islamic Movement), which has carried out a string of attacks inside and outside China. It called the assailants "thugs" and said officials had restored "social order" while continuing to investigate the incident. The ethnic backgrounds of the attackers and their victims were not specified.
Dilxat Raxit, an overseas spokesman for Uighurs in contact with residents of the region, said local security forces have put Pishan under lockdown, with more armed officers guarding roads and residents county residents barred from leaving the area. At least two Uighurs have been detained for sharing videos of the scene, Raxit said.
Authorities typically respond to violence or unrest in Xinjiang with lockdowns, raids on homes, and restrictions on phone and Internet communications.
Calls to local police went unanswered Wednesday, the Associated Press reported.
Pishan county is in the far south of Xinjiang, bordering Pakistan. While China has encouraged Han Chinese to move in Xinjiang as part of broader efforts to settle the region, Han Chinese comprise just 1 percent of Pishan's nearly 300,000 people.