Mack Trucks Inc. announced last week that it will cut production at three assembly plants, effective in mid-November, but the cuts apparently won't affect operations at the firm's Hagerstown, Md., plant.

The company said its production of heavy trucks would be reduced from 154 to 134 daily. The production cuts will be made at Mack plants in Allentown and Macungie, Pa., and Oakville, Ontario.

Mack President John B. Curcio said production adjustment was a normal procedure "reflecting anticipation of traditional reduced industry demand during the months of December, January and February, and the desire to maintain prudent inventory levels."

But despite the cuts, Curcio said the company should have its best year since 1979 "in terms of sales, production and earnings."

Mack posted a $20.1 million profit for the second quarter of 1984, and a record $1 billion in net sales for the first six months of the year.

Sales for the company slumped in late 1979 in the wake of high interest rates and a national recession, but began improving in 1983. Since last fall, Mack has increased the combined production rate at its assembly plants from 79 trucks a day to the current level of 154 a day.

Mack, with about 4,000 workers at its Hagerstown plant, is Washington County's largest employer.