The matinee in the White House briefing room was passing strange. The Assistant Secretary of State for Inter-American Affairs, Elliott Abrams, was telling a group of skeptical reporters that he knows 25,000 Nicaraguans who are "standing around just waiting" for a chance to fight as contras.

But could he tell the whereabouts of the three contra leaders who came here to kick off President Reagan's all-out campaign for military aid and who had just met with him in the Oval Office?

Abrams could not.

He suggested they might be in his office at the State Department. But why should they be there when he was here, he was asked. He trailed off into vaguenesses about their having "gone to the Hill."

Their disappearance could not be explained, either, by presidential spokesman Larry Speakes, who had in the morning promised to deliver them to the news media, under the escort of Patrick Buchanan, who, next to the president, is the best friend the contras have in the White House.

Speakes passed on the remarks of the three rebel chieftains. They had been warmly received by the president and a support group of right-wing notables including the Rev. Jerry Falwell, Phyllis Schlafly and Lewis Lehrman. The president had repeated the familiar alarm about the mortal peril posed by unchecked Sandinista aggression.

"It would mean the consolidation of a privileged sanctuary for terrorists and subversives just two days' driving time from Harlingen, Tex." Hitherto, it has been Brownsville, Tex., that lay across the path of the red tide.

Speakes was having an uphill time fielding questions about certain regional resistance to the plan of giving the contras $70 million in military aid. He was reminded that the foreign ministers of the Contadora countries had, in a recent call on Secretary of State George P. Shultz, voiced unanimous disapproval. "What came out and what was said were two different things," Speakes said opaquely. He was happy to turn the podium over to Abrams.

The problem is, Abrams declared, that the humanitarian aid voted last June is "a completely overt program." The reason the Honduran authorities refused to let it pass through their country is that, like other countries in the area, they are scared of Nicaragua's leftist Sandinista forces.

For years the Reagan administration has contended that U.S. intervention is imperative because the neighbors are secretly demanding it. But something quite different is now being said in public, to the consternation of the White House. The newly elected president of Costa Rica declared that the $100 million in aid to the contras should be sent instead to poor countries in the area.

Why were the plans for the contras' news conference deep-sixed? Did the authorities fear that reporters, in their annoying way, would ask them about ties to the CIA? Arturo Cruz, the most respectable of the three, a banker who was briefly a candidate in Nicaragua's presidential election, admitted that he received CIA money. The same questions would be put to Adolfo Calero and Alfonso Rubelo. The fear is that none of them would remind anyone of Corazon Aquino, the radiant alternative to dictatorship in the Philippines, where the administration suffered the greatest diplomatic triumph of its time in office. Somebody was bound to inquire about contra atrocities.

Another reason for keeping the trio under wraps may have been unresolved differences as to how the contras should be sold to Congress: Are they a desperate, near-prostrate remnant, expiring for lack of U.S. support, or a vibrant, vigorous fighting force poised to rout the Nicaraguan army of 60,000 -- once lawmakers overcome wimpiness and arm them properly? Abrams admitted that the forces have declined, from 8,000 to 6,000 "because of supply problems in the last three months." He got out of it by saying that although small, they are elastic.

He knows for a fact that 25,000 Nicaraguans -- and this includes the 6,000 still in Nicaragua -- are itching to volunteer. Most of them have rifles; give them food and uniforms and they are off to the front.

How does Abrams know this? Where are the would-be warriors? He did not say. He was certain of only one thing about their leaders. They will turn up at the White House later this week, in the company of Vice President Bush.

Are they not under his control? He huffily denied it. But the reason contra aid is such a dicey business is that everybody knows the revolution is one we bought and paid for and, without us, would go away.