The Solicitor General's office, the legal brain trust of the federal government, is in the midst of unusual turmoil, shaken by the announced departure last week of three of its most experienced litigators, by rumors that other senior lawyers are about to leave and by sinking morale.

Current and former lawyers in the elite operation, which argues the government's cases before the Supreme Court, say they are troubled by what they see as an erosion of the office's reputation for objective legal analysis and by a growing suggestion of partisan lawyering.

Uneasiness increased recently when three highly regarded veterans -- deputy solicitors general Andrew L. Frey and Kenneth S. Geller, and senior assistant solicitor general Kathryn A. Oberly -- announced their departures for the Washington office of a major Chicago law firm.

The announcement stunned the 23-lawyer operation, including Solicitor General Charles Fried. Although turnover is not uncommon in the office, rapid changes are unusual at the top. The departure of such mainstays dismayed other career attorneys.

Their leaving, coupled with the abrupt departure in October of 20-year veteran Louis Claiborne, means that only one of the five deputies present when Fried took over last year, Lawrence G. Wallace, is still with the office.

The solicitor general's office has always been something of a hybrid. On the one hand, it is the arm of the Justice Department that serves as the administration's lawyer. On the other hand, it has always enjoyed an unsual degree of political independence. As a result it has had a special credibility with the Supreme Court, whose members see it as a valuable legal resource, in part because of the office's reputation for scholarly analysis.

That special status has eroded, according to some observers, including several former solicitors general, under constant pressure from conservative activists to have the office push the administration's social and political goals through the courts.

Erwin Griswold, solicitor general during the Johnson and Nixon administrations, said recently that he had been "watching the office for 40 or 50 years and had never seen an administration get this involved, intervening so much."

Critics cite the unprecedented decision by Fried to ask the Supreme Court to overturn its 1973 decision legalizing abortion, as an example of an activism that appeases conservatives at the risk of antagonizing the justices, who three years ago reaffirmed the ruling. Another example cited is the Bob Jones University case of 1983, in which the administration challenged the power of the Internal Revenue Service to deny tax exemptions to racially discriminatory institutions.

During this term, according to a Congressional Quarterly study, a record 45 percent of briefs filed by the office have been in cases where the federal goverment was not directly involved. The office offered its views as a "friend of the court" to influence the decisions. That percentage, nearly double the averages of the Johnson and Nixon administrations and a third higher than in the Carter administration, showed "an unprecedented degree of legal activism before the Supreme Court," the magazine said.

In prior administrations, one former agency member said, there was a feeling that "we're better than anyone else," that the attorneys were not political advisers, but "quite different and quite special," working in an office removed from the political fray where the solicitor general's word was virtually never questioned.

That feeling, former office members said, began to fade when conservative activists inside the government consistently tried to persuade the attorney general to overrule President Reagan's first solicitor general, Rex E. Lee. Lee won most of those battles, but in a department run by traditional conservatives William French Smith and his deputy, Edward C. Schmults. The same battles, sources claim, could not be won now with Edwin Meese III at the helm.

Fried's view of the office is more attuned to activism and the administration's agenda. He defends increased "friend-of-the-court" briefs and insists that if he believes, as a matter of law, that a position should be taken and that it is proper for the solicitor general to take it, then it would be dishonest for him not to let the court know his views.

"I think it is less political, not more political," he said in a recent interview, to assess each case as he sees it, regardless of whether a majority of the court is likely to agree.

Politics aside, Fried's handling of the announcement of the recent departures also irritated lawyers familiar with the situation.

Sources said that Fried, after learning of the trio's intentions on March 5, was concerned that the departures would be perceived as a reflection on his stewardship.

He immediately issued a news release March 6 that announced the appointments of three new deputy solicitors general but did not mention the departures. Several sources said Fried saw the announcement as a "preemptive strike" to make it seem that the new deputies were brought in to ease the veterans out.

One source said that while Fried was concerned about appearances, he did not intend to slight the three; he simply assumed their law firm would issue an arrival announcement at the same time.

But others in the solicitor general's office and on the outside were upset by Fried's actions. The resentment was even greater, several knowledgeable sources said, because the trio's departure had little to do with personality or policy differences with Fried.

All three atorneys reportedly had been thinking of leaving for at least some months. Sources said they sometimes found it difficult to work with Fried, and they also may have been uncomfortable with briefs filed under conservative pressure, but they were not resigning in protest.

"It was simply time for them to go," one source said. They had an "offer they couldn't refuse," he added, and an unusual opportunity to continue to work together.