SHANGHAI—Auto sales in China fell for the third straight month in September, as the country’s auto sector confronts what looks likely to be its first yearly decline in passenger-car sales in almost three decades.

Fragile consumer confidence in the face of a falling stock market and U.S.-China trade tensions led to weak sales for most auto makers, as Chinese vehicle sales fell 11.6% year over year to 2.39 million in September, the government-backed China Association of Automobile Manufacturers said Friday.