Sichuan's earthquake relief headquarters activated a level-II response, the Xinhua news agency reported, the second-highest in China's four-tier emergency response system.
Luzhou City launched a level-I emergency response.
The China Earthquake Networks Center said the epicentre of the earthquake was 29.2 degrees north latitude and 105.34 degrees east longitude. It struck at a depth of 10 km (6 miles) at 4:33 a.m. (2033 GMT).
As well as at least three people killed, tens of people suffered light injuries, media reported.