Details of the Commerce Department's "flash" estimate for third-quarter gross national product obtained by the Washington Post show that nearly half the increase in output this quarter is going into business inventories. After adjustment for inflation, nonfarm inventories are rising at an annual rate of about $13 billion in 1972 dollars compared with a decline at a $3.3 billion rate in the second quarter.

Nonresidential fixed investment, which had declined for six consecutive quarters before turning up in the second quarter, rose at a sharp 17 percent rate. Housing construction also rose very strongly despite a drop in home sales during the summer. On the other hand, personal consumption expenditures, after jumping at a 10 percent rate in the second quarter, are increasing at only a 2 percent rate, according to the estimates. Government purchases of goods and services have gone up little this quarter, while net exports continue to be a drag on the recovery.

Some analysts believe that the investment increase already has tapered off somewhat, with the quarterly figures really reflecting business purchases late in the second quarter. At the same time, however, personal spending may be picking up again, with the success of the 1984-model autos holding the key to that total in the short run.