For those readers seeking support for their view that the media have become an arrogant bunch, several members of the White House press corps are cooperating beautifully.

Last week the neophyte White House chief of staff, Donald Regan, met the press to hand out the casual information that the president was making three appointments to his staff. One was Patrick J. Buchanan, columnist, radio and television personality, to be assistant to the president and director of communications. The other two were quickly overlooked.

For the next quarter-hour the guns were trained on Buchanan and the hectoring of Regan began. Regan was reminded of Buchanan's negative views about some of his colleagues in "big media" and was asked, "Why did you appoint him if, in fact, he sees the press as an enemy?"

Regan tried the soft reply, "Well, I don't think he sees the press as an enemy or the media as a whole as an enemy. In fact, he's part of it, being a writer himself and also on the radio and also on television."

The next question was even more blunt, "Why do you need him and what will you do with him?" Regan's reply was low-key: "Well, obviously, he will be a voice, and an important voice, in our communications policies. But you've got to remember that he will be a voice, and his opinion may or may not hold during an entire discussion among quite a few people. That is how policy is set. Policy is not set by one person dictating it; policy will be set by discussions among a lot of us, finally the president deciding on the policy and then going from there."

Then Buchanan's foreign-policy views came under inspection and was the subject of this observation: "I'm wondering how a person who is as clearly identified with one wing or faction within an administration can adequately represent the entire administration and the policies of that administration to the American people."

Regan acknowledged Buchanan "has his views," but said that "in coming with this administration, he agrees that he will support the administration's final position whenever he reflects a position to the public, to the media and in other public utterances."

And if Buchanan should differ with President Reagan's policies, it is obvious that he may find himself ushered into another job or even out the door. Or he may choose to depart, as press secretary Jerry ter Horst did when he couldn't stomach President Ford's pardon of Richard Nixon.

A quarter-century ago, Douglass Cater, reflecting on his years as a Washington reporter and sometime government aide, wrote a book about the Washington media entitled, "The Fourth Branch of Government." There have been some media stars who have acted as if they really were.

How does this come abut? To cover the White House is to ascend to the highest rungs of the journalism ladder. It leads to hobnobbing with the most important officials of government, breaking bread -- and spreading it with caviar -- with kings and prime ministers.

That is a heady atmosphere, and some reporters ease into first-name relationships with the high and mighty; some get into cozy social and recreational associations, and occasionally they are flattered to be asked for their opinion on pending public policies or of people and their potential. Before long, a few journalists take on the airs of high office, even though they have yet to win an election.

While they welcome appointments of one of their own, as, for example, television reporter Bernard Kalb's as State Department spokesman, they found it difficult last week to accept the appointment of maverick Pat Buchanan.

White House correspondents, who often star in radio and television discussions, should certainly realize that one quality that gets them invited back is provocativeness. Columnists know that controversy is often the lifeblood of syndicate salesmanship, and no one has suggested Buchanan was slow in picking up such wisdom. But now that Buchanan has enlisted in the service of this administration, the press corps, and the rest of us, may find he understands his new role, too.

In any case, where is it written that White House reporters have the authority to "advise and consent" in executive-branch appointments?