General Motors Corp. will lay off entire shifts indefinitely at three assembly plants -- one for cars, one for light trucks and one for vans -- and slow down the assembly line by 25 percent at a fourth, GM said yesterday.

The layoffs will add 5,750 to the number of GM employes on indefinite furlough, which now total about 31,500.

Separately, Ford Motor Co. said it would shut down six car assembly plants and two truck plants for one or two weeks starting next week, idling 24,300 employes temporarily. A seventh Ford car plant begins the second week of a two-week shutdown next week, while eight Ford plants start up again from short shutdowns.

Meanwhile, Chrysler Corp. said it will suspend production for one week at two of its car assembly plants and one truck plant to control inventories. The move will idle a total of 9,200 hourly workers for one week.

The automaker said the plants affected in the closings are its assembly plant at Newark, Del., which makes compact models, its assembly plant at St. Louis, which makes intermediate-sized cars, and its Missouri truck plant near St. Louis, which makes light-duty trucks.

And an auto industry publication reported that U.S. auto production will increase almost 10 percent this week compared to a week ago because Ford Motor Co., reopened four plants and several local strikes at General Motors plant ended.

Automotive News reported that planned output this week is 180,920 cars compared with the 164,844 built last week.

But this week's projected total is down 19.18 percent from the 223,860 cars assembled this week a year ago, the trade publication said.

Truck production is scheduled to fall 10.68 percent this week, from 50,478 last week to 44,998. That compares to the 87,937 trucks built this week a year ago, almost twice as many.

At Ford, three car and four truck plants are down this week to balance inventories. Four GM plants are slated for overtime, while two Chrysler Corp. plants worked nine-hour shifts during the week as did the Volkswagen of America plant.

U.S. car production for 1979 totals 7,599,676, down 5.02 percent from the 8,001,103 built through Nov. 11 last year.

In Canada, car production this week remained consistent as 21,636 units were produced compared with 21,753 cars last week. There were 24,185 cars built in Canada in the same week a year ago. Year-to-date car production in Canada totals 861,527 compared with 989,364 during the same period in 1978.

U.S. truck production for 1979 totals 2,779,631, a drop from the 1978 total of 3,279,389 during the same period. Canadian truck production this week was up, totaling 11,744 compared with 10,492 last week. There were 14,723 trucks produced in Canada during the same week a year ago. Year-to-date Canadian truck production totals of 573,080 compare with 576,472 a year ago.

The GM plants losing their second shifts are the Lakewood, Ga., light-truck plant, where 700 workers will be dropped on Dec. 3; the van plant at Lordstown, Ohio, where 1,250 will be let go on Nov. 26; and the car plant at Doraville, Ga., which makes Chevrolet and Oldsmobile intermediate cars, where 2,300 will be let go on Nov. 26.

The production at the Norwood, Ohio assembly plant, which builds the Pontiac Firebird and Chevrolet Camaro, will slow down from 56 cars an hour to 42 on Jan. 2, dropping 1,500 workers in the process.

GM this week added a second shift to its compact-car plant at Oklahoma City and has scheduled a speedup in subcompact production at Lordstown from 80 to 90 cars an hour for Jan. 2.

Ford said its Kansas City and Wixom, Mich., plants would close for one week starting Monday, with furloughs for 2,400 at Kansas City and 5,100 at Wicom.

Two-week shutdowns were scheduled for Ford car plants at Chicago, with 3,700 workers; Mahwah, N.J., with 3,300; St. Louis, with 2,300; Wayne, Mich., with 3,800; and the van plants at Lorain and Avon Lake, Ohio, with 3,700 workers. The Metuchen, N.J., car plant starts the second week of its two-week shutdown next week.

Ford plants starting up next week were car plants at Los Angeles, Louisville, Ky., and Oakville, Ontario, and truck plants at Kansas City, San Jose, Calif., Minneapolis, Louisville and Oakville.