Montgomery County's police chief said yesterday he plans to abolish a list that governs promotions for about 640 of the 770-member force because at least three officers cheated on the 1975 examination that determined rankings.

Chief Robert J. DiGrazia said that although he has proof that only three officers cheated, he has "some concern" that the cheating was more widespread, and that the current promotional list is therefore "tainted."

The chief's decision leaves the department with no current system for promotions and promptly brought some complaints.

"I studied six months for that (promotional) test and I've been number two on the list since the chief got here... Because he (diGrazia) found those three cases of cheating, he's saying it was everbody. I didn't cheat," declared Sgt. Thomas Lowther, who took the exam for second lieutenant.

The chief said he also decided to abolish the current promotional list because the department is plaining to implement a new "career development" program that would change both the type of promotional examinations used and the procedure for selecting a police officer for a promotion.

The list has been in effect since January 1976, and so far 68 officers have received promotions, according to police spokeswoman Nancy Moses. Normally, a list becomes outdated affter 15 months. But diGrazia, at the urging of some of the police officers, previously decided to extend use of the current one because the testing system for promotions under the new career development program had not been fully worked out.

Cpl. Gerald A. Boone, president of the county's police association, criticized the chief for failing to keep the promotional list until new promotional procedure could be put into effect.

The department has applied for a $55,000 grant to hire a consultant who would develop new promotional tests. If the department gets the grant, it would take five to six months to develop the test, according to Stephen Gaffigan, director of research and planning for the department.

The County Personnel Office would then have to approve the tests, a process that could cause additional delays on installing any new system for promotions, Boone said.

Boone added that he will appeal the chief's decision to the personnel office. "I would hope other members of the association would file personal appeals as well," Boone said.

DiGrazia said yesterday he plans to hold a general testing sometime this summer of in early fall for officers who want to become sergeants because, under the new career development program, "We need sergeants - supervisory personnel."

"He's talking about giving tests to corporals for the rank of sergeant. What about the rest of us?" Lowther asked.

Not all police officers were critical of DiGrazia's move, according to Lt. Donald Hearn. "He's not trying to punish the department ... he's looking at a better way to go" with promotions, Hearn said.

"The basic feedback (from police officers) is that they want to get on with the show, have new tests and a new list," Hearn added.

The new examinations, according to Gaffigan, will contain questions relating to the particular position for which an officer is applying. "They will not contain a whole series of questions covering every job in the police department," he added, as the old tests did.

Under the old system, an officer applying for a sergeants position would often take the same exam as a lieutenant seeking a captain's rank.

The police cheating scandal involved officers who had apparently been told what test questions to expect by officers who had already taken the exam.

As a result, the chief disciplined three first lieutenants last month. One officer was demoted to second lieutenant and ordered to pay back the money he earned as a result of his March 1976 promotion to first lieutenant. Another officer was also demoted to second lieutenant and directed to return half the money earned as a result of his September 1976 promotion.

A third officer, who was not promoted as a result of the 1975exam, was ordered to take a one-step pay reduction for six months.