Three Montgomery County police lieutenants have been punished with demotions or less pay for cheating on a 1975 promotional exam taken by more than 600 officers.

A police source confirmed that the three men, all first lieutenants, are Don Hearne, Tom Rufty and John Crawford.

Rather than seek departmental hearings on the charges, the three officers accepted the punishment ordered April 7 by Chief Robert diGrazia, a police spokesman announced yesterday.

The discipline followed a six-week investigation during which a dozen police officers were interviewed by the department's Office of Internal Affairs about alleged cheating.

A police spokesman emphasized during the investigation that cheating violates the department's internal rules and is not a violation of either state or county law.

Chief diGrazia declined to disclose the names of the officers, saying the matter was an internal one, and the spokesman who outlined the charges against each man and the punishment he faces refused to supply the identifications.

One of the officers was charged with "unauthorized receipt of test questions from immediate subordinates in a prearranged meeting," he said. This man was demoted from first to second lieutenant and ordered to return the money earned as a result of his March, 1976, promotion to first lieutenant.

A second officer was charged with "unauthorized receipt of test questions in a prearranged meeting," the spokesman said. He was demoted from first to second lieutenant and ordered to return half the money earned as a result of his September, 1976, promotion.

A third officer was charged with "unauthorized receipt of test questions and failure to take proper supervisory action" when he learned that the test information was being transferred. This officer, who was not promoted as a result of the test, was ordered to take a one-step pay reduction for six months.

The annual base pay of a first lieutenant is about $23,700 some $1,000 more than the base pay of a second lieutenant, according to police spokesman Phil Caswell.

But sources said the three officers were earning closer to $30,000 a year, and reduction in rank would probably mean a salary cut of about 5 percent.

Talk of possible cheating on the 1975 tests was termed by one official last February a "continuing source of annoyance in the department" during the past two years.

Caswell said yesterday that the disciplining of the three officers, "as far as I know, ends the investigation" by the Office of Internal Affairs.