The three people President Reagan named to the six-member Civil Rights Commission last Wednesday wouldn't have been the choices of the country's oppressed minorities. All three oppose affirmative action as it is generally understood.

But that's not what is most disturbing about the long-rumored changes at the commission. What is profoundly disturbing to those who have followed and supported the work of the commission is that Reagan has changed not just its makeup, but its very nature.

The genius of the agency, since its Eisenhower-era inception, has been its taken-for-granted independence--even when its reports and findings and recommendations infuriated and embarrassed presidents. The commission's role has been that of keeper of the mirror that reveals the specific ways in which our private and public leadership has fallen short of our democratic ideals. It hasn't always been right, but its notions have always been its own, unaffected by the political requirements of a particular president.

Reagan has changed that by firing half the commission--the first time any president has fired any of its members. President Nixon came close in the case of Father Theodore Hesburgh, who, after serving under four presidents, finally was pressured into resigning. But Hesburgh wasn't fired. Reagan, breaking precedent, has fired Mary Berry, Rabbi Murray Saltzman and Blandina Cardenas Ramirez--apparently because he disliked their attitude, their point of view and their criticism of some of his turn-back-the-clock approaches to civil rights.

Speaker Tip O'Neill called the firings "a political purge of . . . an institution that has long stood as a fair and independent body on behalf of civil rights protection in our country."

And so it seems to me. This is no denigration of the men chosen to replace the fired commissioners--Morris Abram, John Bunzel and Robert Destro--all of whom insist that they are "independent minded." Perhaps they are. But there is a difference between having a politically independent commission, as the CRC indisputably has been, and having independent-minded members who understand that their tenure depends on pleasing the president.

As vacancies occurred, other presidents named to the commission people who reflected their own views. What Reagan has done, by his firings, is to transform the commission from a vital "mirror" into just another tractable arm of his administration.

If he had been able to do to the federal courts what he has done to the commission, he might have avoided an embarrassing string of setbacks for some of his more controversial policies, including the latest comeuppance: the 8-to-1 Supreme Court ruling that he was wrong in his desire to allow federal tax breaks for the racially discriminatory Bob Jones University and Goldsboro Christian Schools.

If the newest appointments are confirmed by the Senate, five of the six members of the CRC will be Reagan appointees, including the conservative and controversial Clarence Pendleton, the only black member. Until recently, White House plans had included dumping the sixth non-Reagan appointee, Jill Ruckelshaus. Their second thoughts may have been prompted by the fact that her husband has recently been asked to take over and rehabilitate the disgraced Environmental Protection Agency.

Reagan, like any president, is entitled to his own views and to make his own appointments. But his treatment--his transformation--of the Civil Rights Commission is little short of contemptuous of the interests of America's minorities.

And just the other day, he was complaining that he has been a victim of a "perception" problem among blacks.