The pace of gross domestic product growth followed a strong 4.6 percent expansion in the second quarter, when the world's largest economy rebounded from a first-quarter contraction linked to unusually severe winter weather.
Economists had expected third-quarter growth would slow even more, to a rate of 3.0 percent according to the average estimate.
In its first estimate for the July-September period, the Commerce Department said the deceleration largely was due to slower business investment from the second quarter, when an increase had helped boost growth.
Consumer spending, the key driver of the world's largest economy, also cooled, rising 1.8 percent after increasing 2.5 percent in the prior quarter.
Disposable personal income grew 2.7 percent, down from a 4.4 percent rise in the second quarter.