Three federal agencies moved yesterday toward a major change in food labels to tell buyers more about the main ingredients, preservatives, flavorings and other contents of the foods they buy.

For example, the agencies want to make all manufacturers tell buyers how much salt, sugar and fat are in many foods, and how much beef there is in "beef stew" or chicken in a "chicken pie."

The agencies -- the Food and Drug Administration, Agriculture Department and Federal Trade Commission -- released a 177-page plan to achieve such goals. Officials will wait 90 days for public comments, then prepare regulations they will publish. After that there must be another comment period, and official consideration, before final regulations are issued.

This process could consume much of 1980. Still Assistant Secretary of Agriculture Carol Tucker Foreman said yesterday's move represents "a major government-wide effort to improve food labeling" and "the most important action in the area in over 40 years."

The labeling would affect most of the food in markets, except fresh fruits and vegetables and fresh or frozen, otherwise unprocessed, meat, poultry and fish.

Consumers, she said, "clearly have a 'right to know'" exactly what they are buying and eating, and "should not have to be scientists, economists or make use of a magnifying glass or ouija board" to decipher the fine and confusing print that often appears today on food packages.

In addition to making new regulations, she and other officials added, they will seek important changes in food laws where they now lack authority -- for example, to order full declaration of ingredients in standardized foods like bread and cheese, which may actually vary greatly from variety to variety.

Many food manufacturers have voluntarily begun some of the kinds of labeling the federal agencies want. But consumer groups have been pressing the FDA and the Agriculture Department for years to see that consumers know exactly what they're eating.

They have said, for example, that diabetics and dieters often don't know there is sugar in a prepared product. People with hypertension, who must avoid salt, often are unaware of the undisclosed presence of salt, they said.

The agencies said they expect to propose:

The declaration of "optional" ingredients, like sugar or additives, in standardized foods like bread and cheese.

The declaration of sources of fats and oils in foods containing 10 percent or more fat before processing.

A statement on labels telling consumers ingredients are listed in descending order by dry weight. They are so listed now, but many people don't know it.

The disclosure of sodium (including common table salt), potassium and sugar content, as well as disclosure of total calories, fat, carbohydrate, protein and cholesterol when nutritional claims are made.

Mandatory disclosure of the amount of meat or poultry in products where they are supposed to be the "significant" ingredient, and mandatory dating of perishable and semiperishable meat and poultry products, like bacon and sausage.

FDA and Agriculture officials said they will seek legislation to dictate even fuller disclosure of ingredients, as well as more nutrition labeling. They said they still have not decided -- and want public comment -- on what information the public wants on nutrition, and what rules to make for imitation and substitute foods, like imitation "cheese" and "eggs."