WASHINGTON - The Commerce Department reported the economy grew at a 1.5 percent annual rate in the third quarter, a sharp slowing from the 3.9 percent rate reported for the second quarter. The falloff was mostly due to slower inventory growth. Inventories subtracted 1.44 percentage points from the growth rate in the quarter after adding 0.02 percentage points in the second quarter. Final demand for the quarter grew a 3.0 percent annual rate. For the first three quarters of the year, GDP has risen at a 2.0 percent annual rate.