Research firm Gartner said this was the 12th consecutive quarter when shipments had trended downwards.
The figures are preliminary estimates and final statistics are expected soon.
While HP Inc and Lenovo shipped almost the same number of units during the quarter. HP was in an upward trend as this was its fifth consecutive quarter of growth.
Gartner principal analyst Mika Kitagawa said that while there were signs the PC industry was stablilising in regions like EMEA, Japan and Latin America, these were offset by the US market, which experienced a 10% year-over-year decline, partly because of a very weak sales during the back-to-school season.
Data includes desk-based PCs, notebook PCs and ultramobile premiums (such as Microsoft Surface), but not Chromebooks or iPads.
"Business PC demand, led by Windows 10 upgrades, continued to drive PC shipments across all regions, but its refresh schedule varies by region," she said.
"The countries with stable economies, such as the US, have created a positive sentiment among businesses, especially for small- and mid-sized businesses, which are more vulnerable to external events, such as economic or political."
Gartner also noted that there had been continuous shortages of components like DRAM, which had gotten progressively worse in the third quarter when compared to the first quarter of 2017.
"The component price hike impacted the consumer PC market as most vendors generally pass the price hike on to consumers, rather than absorbing the cost themselves," Kitagawa said. "We expect the DRAM shortage to continue to the end of 2018, but it will not be reflected in the final PC prices immediately."
HP Inc saw growth in all key regions except in the US market. Lenovo experienced its steepest fall in US shipments year-over-year since it bought IBM's PC division in 2005.
Gartner said Lenovo appeared to be caught between increasing market share or profits and looked to be placing more emphasis on profitability.
Dell, meanwhile, saw slightly lower PC shipments globally, its first fall since the first quarter of 2016.
In the Asia-Pacific region, PC shipments reached 24 million units, down 2.1% from the corresponding period in 2016.
Consumer demand was lacklustre but PC demand among businesses remained steady, especially notebooks.
In China, a decline of 5% was seen. Shipments were more stable in the business market, particularly for large enterprises, than for the home market.
Graphic: courtesy Gartner