A newly triumphant group of South Korean generals showed their political muscle today by having three of their choices appointed to powerful Cabinet seats in the weakened civilian government of President Choi Kyu Hah.

The posts of minister of defense, justice and home affairs went to two generals and a former prosecutor. The appointments, according to reliable sources, had been demanded by the new military leaders and greatly expanded military control over civilian affairs.

It was the first clearly political move by the faction that gained control Wednesday by suddenly arresting the former martial law commander.

Choi, who has remained silent about the military takeover, reemerged in public today, and political sources said he expects to continue in office.He is still scheduled to be formally inaugurated next week.

But Choi's power has clearly diminished since the new generals took over the military Wednesday. Sources said that three names on his original Cabinet list were removed and the choices of the generals inserted in their place.

Choi had no comment when the final list was made public by a spokesman. One of his first public duties since the takeover was pinning a fourth star on the new martial law commander, Gen. Lee Hui Sung, who also is the choice of the insurgent generals.

Choi was badly shaken by the arrest, after a gun battle, of the former martial law commander, Gen. Chung Sung Hwa, because Choi has staked his brief administration on appeals for order and stability after succeeding the assassinated president Park Chung Hee.

The military leaders at first claimed they had Choi's approval for the arrest of Chung. Reliable sources said his assent was demanded by the generals but may never have been given.

It remains to be seen how far Choi, a career bureaucrat, will be allowed to go in his other moves to liberalize a political system inherited from Park's days. He had sought cooperation from opposition members, abolished an antidissent decree, freed many political prisoners, and promised to preside over a government that would amend the constitution to provide for free, direct presidential elections.

The ascendant generals have made no public comment on those moves. Some observers familiar with their opinions think they will restrain Choi but will not insist on retaining Park's constitution. A spokesman yesterday said the new leadership does not favor keeping that constitution.

Most observers think the new leadership is in a position to get whatever it wants because the military has far expanded its influence since Park's assassination. Much of it was accomplished by Chung and his associates before they were overthrown and the new group has added new power.

Besides the military bureaucracy, the generals now control the Korean Central Intelligence Agency, with its vast surveillance powers, and the Martial Law Command, which severely punishes those who demonstrate against it.

With today's Cabinet switches, the generals control the most powerful three ministries except for those involved in economic affairs, over which Choi was permitted to exert his own authority.

The Home Affairs Ministry controls the large national police force and provincial governments and administers election laws. Choi's choice to head it was dropped in favor of Gen. Kim Chong Hwan, who was chairman of the joint chiefs of staff.

Park Sang Gi, a former government prosecutor and a close ally of the late president, was named to head the Justice Ministry, which prosecutes all criminals, including demonstrators and dissidents, when martial law is not in effect.

The most important switch imposed on Choi was the Ministry of Defense. Choi had planned to keep Gen. Ro Jae Hyun, the current minister, who had worked closely with Choi since the assassination. Instead, the position will be filled by Gen. Choo Young Bok, the former Air Force chief of staff.

Neither Choo nor Kim was involved in the sudden military takeover by generals who are their juniors in rank and years of service. But they were put in the new posts by pressure from the younger generals on Choi and their old posts will be filled by representatives of the insurgents, according to well-informed sources.

The rest of the 18-member Cabinet apparently reflected Choi's wishes and an attempt to appeal to groups and elements of the public excluded from government under his predecessor.

For the first time a member of the opposition New Democratic Party was given a Cabinet position and the Ministry of Education went to Kim Ok Kil, a renowned educator who recently retired as president of Ehwa Women's University. She is the country's third woman to serve in the Cabinet.

Choi also succeeded in appointing his choice as deputy prime minister, usually the country's top economic planning post. He named Lee Han Bin, a professor and dean of a technical college.

Meanwhile, courts-martial of those accused in the Park assassination plot resumed today after a day's postponement, without any word about the fate of Chung, the former martial law commander.

His arrest for interrogation was allegedly the result of new evidence of his participation by the accused assassin, Kim Jae Kyu. The generals who arrested him and seven or nine other generals have so far not disclosed that alleged evidence.