The nation's economy slowed more abruptly in the third quarter than previously thought, the Commerce Department said yesterday, and economists saw little chance of a rebound to levels the Reagan administration said would help reduce the huge looming budget deficits.

The Commerce Department initially had estimated that the inflation-adjusted gross national product rose at a 3.6 percent annual rate in the third quarter and later lowered that to 2.7 percent. Yesterday, it said the actual increase was only at a 1.9 percent rate. The economy hasn't been that sluggish since GNP grew at a 0.5 percent rate in the fourth quarter of 1982.

Some economists, including an informal adviser to the president, said there is no evidence -- particularly in light of yesterday's report -- that the economy can pick up enough speed to reach the 4 percent growth rate that the Reagan administration has forecast for next year.

In addition, economists said that a weakening economy narrows the policy choices available to the administration and Congress for dealing with the large budget deficits, because tax increases and spending cuts tend to slow the economy even more.

However, the administration remained upbeat and said it expected a rebound from a temporary lull sometime next year.

The Commerce Department also reported yesterday that housing starts fell 9.8 percent in October to an annual rate of 1.5 million units, the lowest level since December 1982, and corporate after-tax profits dropped 7.3 percent in the July-September quarter, following a 0.3 percent drop in the second quarter.

The Commerce report yesterday blamed the slower growth in the third quarter on a slowdown in consumer spending and a large decrease in net exports. But economists said growth may not be much better in the fourth quarter.

"It's evident from what's going on in the fourth quarter that growth is still stalled," said Alan Greenspan, an informal adviser to the president. "There would have to be some fairly significant pickup very soon to get back on track" to the pace the administration expects.

"The data are unambiguous," said economist Allen Sinai. "They tell us a growth recession and maybe something worse. All of these numbers were lower than the lowest expectations of the economy."

Some economists have said the economy already has entered the first phase of a growth recession, when output increases too slowly to prevent the unemployment rate from rising.

Economists blamed the increasing sluggishness of the economy on high interest rates, which they said have helped to dampen consumer spending on durable goods such as major appliances and furniture as well as on housing.

In addition, the high interest rates attract investment from overseas. That tends to push up the value of the dollar, consequently making imports cheaper relative to domestic goods and dampening exports. The intense price competition between domestic firms and foreign suppliers has kept prices low and forced profits down, economists said.

Greenspan said that, if the trade picture had not deteriorated from the second quarter to the third, the gross national product would have risen at a 5.7 percent rate.

Some economists also blamed the tight-money, anti-inflationary policies of the Federal Reserve Board for the slowdown. They said that the Fed's restraints on the growth of the money supply earlier in the year led to the slowdown of economic activity in recent weeks.

With the economy moving sluggishly, the Reagan administration and congressional policy makers are placed in a tight spot, economists said. Slow economic growth generally leads to more unemployment, which increases government outlays and reduces tax collections and subsequently increases the federal budget deficit.

During periods of sluggish growth, tax increases -- which would reduce the budget deficit -- usually are ruled out because they would tend to retard growth even more. Many economists recommend tax increases during boom times, such as earlier this year or last year.

A reduction in spending also tends to cost jobs and reduce incomes, which lowers consumption and dampens the economy.

But if the federal budget deficit is not reduced, many economists expect the government's borrowing costs to rise, further increasing the deficit.

"The weak economy makes it very, very difficult to do anything about the deficit," Sinai said. "There's a one-in-four shot this episode could turn into a recession."

The administration's budget-cutting targets depend on a real growth rate of 4 percent annually through 1988 and gross national product increasing to about $5 trillion by 1988. GNP was at an annual rate of $3.7 trillion during the third quarter.

Greenspan and Commerce Department Chief Economist Robert Ortner both said a recession doesn't seem likely. They said that business inventories are not excessive, easing the threat of production cutbacks by manufacturers, and interest rates are declining, while the nation's industries still have room for noninflationary growth.

Ortner said consumers still are spending, though at a slower rate, and their incomes are increasing, factors that point toward future growth. In addition, he said that business investment is still strong, and that he expects the trade picture to improve next year.

A 4 percent growth rate for next year "I think is still in the cards," particularly if inflation stays moderate, Ortner said. Commerce reported yesterday that inflation as measured by the implicit price deflator, which measures changes in prices and the composition of output, increased 3.7 percent in the third quarter, compared with 3.3 percent in the second quarter.

Business investment grew at a 15.9 percent rate in the third quarter, compared with a 12.3 percent rate in the second quarter. Personal consumption expenditures grew at a 0.6 percent rate, compared with 7.9 percent in the second quarter.