Chrysler Corp. will lay off 3,500 salaried workers by the end of this year and close at least one of its 10 U.S. assembly plants by 1990, company officials said yesterday.

Chrysler Chairman Lee A. Iacocca said the cutbacks are needed to help reduce operating costs associated with Chrysler's takeover of American Motors Corp.

The cuts also are needed to help Chrysler compete in what many U.S. auto industry officials expect to be a recessionary economy in 1989, Iacocca said.

"The timing of the AMC merger is turning out to be fortunate for us in light of what's happening on Wall Street, because it forced us to focus on ways to increase revenue and cut costs," Iacocca told reporters at a suburban Detroit press conference, where he announced third-quarter corporate earnings of $253 million ($1.15 a share).

The nearest Chrysler plant to the Washington area is an assembly plant in Newark, Del., where about 4,200 people are employed. The plant produces Plymouth Reliant America and Dodge Aries America sedans, as well as Chrysler LeBaron passenger cars and Town and Country station wagons. But it is not likely that Chrysler's Delaware employes will be affected by the planned layoffs, company sources said.

Chrysler's third-quarter earnings were 7.7 percent higher than its $235 million ($1.06) in net income for the year-ago period.

General Motors yesterday reported a third-quarter net income of $812 million, up from $345.1 million in last year's quarter.

On a per-share basis, GM's third-period earnings worked out to $2.28 for GM common; 68 cents for Class E shares, earnings attributable to the performance of GM's Electronic Data Systems subsidiary; and 85 cents for Class H shares, attributable to GM Hughes Electronics Corp. Comparable figures for the third-quarter 1986 were 80 cents for GM common, 58 cents for Class E and 82 cents for Class H.

But GM's seeming fiscal triumph largely was brought about by a change in the way the company figures depreciation costs on its plants and equipment. In the past, GM often wrote them off within a year of when they were incurred. Under its new system, GM is spreading those costs out over a longer time, as long as three or four years in some cases.

Had GM stuck with its old depreciation-cost accounting, the company would have posted a $500 million operating loss in its main business for third-quarter 1987, according to some auto industry analysts, rather than the $356.8 million operating net that GM reported. GM officials defended their accounting change, saying that it reflects "more realistic assumptions regarding the useful lives of GM's plants and equipment and special tools."