Both IDC and Gartner are out with their reports for PC shipments in the third quarter today. While Apple is not in the top five vendors for worldwide shipments, estimates from the two firms place Apple’s share of the United States market at 12.5-percent to 13.6-percent.

According to IDC’s numbers (above), Apple captured 12.5-percent of U.S. PC shipments in the third quarter. This is up from 11.8-percent in the same quarter last year. Apple faced a year-over-year unit decline of 7 percent, but market leaders HP and Dell posted shipment declines of 18.8-percent and 16 percent, respectively. Meanwhile, shipments for the U.S. market in total were down 12.4-percent.

In comparison, the same numbers from Gartner have the total market decline at 13.8-percent. Apple hit 13.6-percent of the U.S. market, where as it had a 12.5-percent estimate for Q3 2011. Estimates from both firms put Apple’s market position firmly behind HP and Dell, despite IDC estimating fourth-place Lenovo’s growth at over 9 percent for the quarter: