Well, we now know what to do with ex-presidents. We send them to dangerous, world-class funerals. Richard Nixon, Gerald Ford and Jimmy Carter were entirely gratified to go to Cairo--to ride again on Air Force One, to feel wanted, to be cosseted, consulted, interviewed and photographed as of old.

" . . . I know the three of us feel that being brought together on this occasion was in the best interest of the U.S., and all of us in the future, if asked, would do exactly the same," said Gerald Ford on the flight home.

Does that mean that other countries whose leaders have the misfortune to be assassinated will expect the trio to be present at the funerals? If so, what will we do with the vice president, who usually represents the chief executive at such melancholy occasions?

George Bush has been exceptionally self-effacing. Democrats gloated that he was conspicuously benched during the struggle over the tax cuts on Capitol Hill. That was, of course, because during his two-year pursuit of the presidency, he said only two memorable words--"voodoo economics"--to describe President Reagan's fiscal policies. The phrase pursues him like a cur. The day that the three former occupants of the Oval Office set out for Egypt, he was speaking at the National Press Club. His subject was a safe one--AWACS--but inevitably, in the introduction, "voodoo economics" was mentioned.

Bush was never going to be a Spiro Agnew, hammering home presidential points until they hurt. You cannot imagine him calling students who might complain of deprived college loans "an effete corps of impudent snobs," or Wall Street tycoons "nattering nabobs of negativism."

He has not been embroiled in a public brawl with old friends, as was Fritz Mondale at a comparable time in his vice presidency. Mondale was sent to the Senate to put down a liberal filibuster on natural gas deregulation.

But a number of people in the Press Club audience thought that Bush should not be there, that he should be headed for Cairo to pay a last tribute to the bravest of his contemporaries, Anwar Sadat.

Bush explained that neither he nor the president had gone "for security reasons." He said that "our friends in Egypt understand that." The nervous new government in Cairo might have understood it even better if Menachem Begin, the most wanted man in the Arab world, had not shown up --and walked behind the coffin.

It isn't the first time a president has declined to send a vice president to a major funeral when he couldn't go himself. Lyndon Johnson wouldn't let Hubert Humphrey go to Winston Churchill's.

"The spectacular delegation" we had sent, Bush said, "does honor to Sadat."

How much honor a delegation of which Richard Nixon is a member does to the United States is a question. Our only resigned president shamelessly seized the occasion to do himself even further honor, by embarking on a "private" tour of the troubled region. He seeks to advance the political rehabilitation that Jimmy Carter began by inviting him to a Chinese dinner at the White House. He has made himself what he always thought he ought to be, a roving ambassador for peace.

Presidential counselor Edwin Meese III said on "Issues and Answers" that the White House will be "very much interested in what he learns."

We do not know if Nixon sought the permission of his fellow-traveler on Air Force One, Alexander M. Haig Jr. The secretary of state, in the final hours of Watergate, steered Nixon to the exit. Did he clear him for takeoff to Saudi Arabia?

The old lines of authority are perhaps blurred by now, as are our old hostilities--and our post-Sadat Mideast policy.

The three former presidents are all friends presently, and friends of the current occupant. Jimmy Carter comes to the White House to stand by Reagan's side to promote the AWACS sale, with which the other two agree. And Carter and Ford, who could not think of a thing to say during the long power failure at their first debate in 1976, could not stop talking to each other and held an extraordinary joint news conference on the flight home.

It may be that it is "in the best interests of the country" to have a show of comity among the three ex-presidents. But what was most striking was their out-of-office outspokenness, especially on direct U.S. talks with the PLO, a total no-no in their White House days.

"At some point that has to happen," said Ford.

Carter chimed in, "I think that Jerry is certainly right in saying these discussions have to be done."

Carter, of course, had to fire his U.N. ambassador, Andrew Young, for secretly meeting with the PLO in 1979.

It may be that it is valuable to have ex-presidents saying things that would cost a vice president his evermore thankless job.