Three Democratic congressmen and 19 medical and consumer groups pressed the Reagan administration yesterday to make food firms reveal the amount of salt in hundreds of products.

Salt and other forms of sodium, such as monosodium glutamate, are used in copious amounts in many foods. But 35 million Americans with high blood pressure who are harmed by excess salt and a far larger number who may develop high blood pressure if they eat too much salt have no way of knowing the amount.

To repair this -- and "promise the health of millions" -- Reps. Albert Gore Jr. (Tenn.), Henry A. Waxman (Calif.) and Neal Smith (Iowa) urged the administration to act on its own to require salt labeling on processed foods and at the same time introduced a bill to require such labeling.

Backing them were 19 groups ranging from major medical organizations such as the American Public Health and American Heart associations to activists such as the Center for Science in the Public Interest.

The American Medical Association has taken a similar position but did not join the other groups in a letter urging action upon Health and Human Services Secretary Richard S. Schweiker.

The Smith-Gore-Waxman bill, described by Gore as "a minimalist bill" to regulate as little as possible, would require disclosure on canned and processed foods, or on signs in stores, of average sodium and potassium values over 35 milligrams a serving. Food firms with less than $500,000 a year in sales would be exempt.

The average person needs only 200 milligrams of sodium (a tenth of a teaspoon) daily, but the average American consumes 4,000 to 10,000 milligrams, a third of it from processed foods, Dr. James Bernstein of the American College of Preventive Medicine reported.