MINNEAPOLIS, JAN. 7 -- Pillsbury Co. said today its board of directors approved a major restructuring plan that includes dropping three of its five full-service restaurant chains, resulting in a fiscal third-quarter charge of $91 million.

Pillsbury said it would sell or close all of its Quik Wok, Bay Street and Key West Grill restaurant chains, which totaled about 70 units at the end of 1987.

When the changes are complete, Pillsbury will be left with only two full-service restaurant chains -- Steak and Ale, with about 135 units, and Bennigan's, with about 200 units.

At the end of fiscal 1987, the company operated 411 full-service restaurants under five names. The fifth full-service chain was Innovative Food Services, a fledgling chain of hotel restaurants.

With the shutdown of Quik Wok, an experimental Asian fast-food chain, Pillsbury will be left with Burger King and Godfather's Pizza as its only fast-food enterprises. There are about 5,300 Burger Kings and about 600 Godfather's Pizza outlets.

The company said it will sell or close 28 low-performing company-operated Godfather's Pizza outlets and 15 Burger Kings.

In addition, Burger King will upgrade 145 units, primarily in the Chicago and New Orleans markets, and modernize or close about 50 company-owned restaurants operated by franchisees.

"This represents a major change in emphasis for our Restaurant Group," Chairman John M. Stafford said. "By concentrating on fewer concepts, we will capitalize on our strengths and improve our overall performance."

Pillsbury spokesman Johnny Thompson said the charge, which amounts to $1.06 per share, will result in a net loss for the third quarter ended Feb. 29.

However, Stafford said the restructuring was designed to strengthen the competitive base of the company's core business. Pillsbury posted net earnings in fiscal 1987 of $182 million.

In composite trading on the New York Stock Exchange today, Pillsbury closed down 62.5 cents at $37.25. The stock had risen $1.62 1/2 on Wednesday in anticipation of the restructuring announcement.

Thompson said the company does not know how many jobs will be lost in the reorganization. "We expect some consolidation over time and we haven't made those decisions yet," he said.

In the consumer foods division, two manufacturing plants in the United States will be closed or sold. The company did not identify the plants.

Pillsbury said it also will close or sell two small canned vegetable businesses acquired with Joan of Arc in 1985 and two small International Foods businesses.