Served up like the star attractions in a Hollywood extravaganza, three young Americans tonight acquitted themselves with all the aplomb of veteran performers.

Ushered onto a stage arranged by their radical Islamic student captors at the U.S. Embassy, the first three of 13 American hostages who were to be released later showed little outward sign of their two-week ordeal as they answered reporters' questions for 1 1/2 hours while still under guard by three captors.

Kathy Gross, 22, a secretary form Cambridge Springs, Pa., owed her release to being a woman and the two men, Ladell Maples of Earle, Ark., and William Quarles of Washington, to being black. Maples and Quarles, both 23, are Marine security guard sargeants.

Iran's ruler, Ayatollah Ruhollah Khomeini, yesterday decided to free the women and blacks "provided they were innocent of espionage." It was just before 9 p.m. when the three made their way through the ranks of students, who were singing a revolutionary song hailing Khomeini and shouting "Allah Akhbar" or "God is great."

Outside in the street, the crowd kept up a seemingly nonstop chant: "Death to Carter, death to the shah."

Several of the captors organized the questions -- at first limited to a single query from each local and foreign journalist -- and the three fielded even the most difficult subjects with seeming ease.

Little of what they said was new. The conditions of their difficult detention had become known earlier through various diplomats' visits to the 27-acre U.S. Embassy compound.

But it was nonetheless moving to hear Gross first explain that she had enjoyed "very good conditions" of detention only to admit later on that her most difficult problem was "spending 16 hours a day with your arms tied to the arms of a chair."

The two Marines generally agreed with her verdict that they had been "fed more than fairly, slept nights and had no problems physically."

She mentioned, however, that perhaps "people were mentally upset."

The hostages are "very upset and don't know what's going on," she said. "They cannot communicate with each other and aren't getting any news." She was kept in a building with the six other women hostages.

The two men were kept in isolation and were not allowed to communicate even when physically in the presence of other hostages.

Quarles said, "I mostly stayed in the living room of the house and most of the time it was just me. If they wanted to switch someone (from room to room), they would move me so I wouldn't see them."

Maples said, "I was tied up the first day," and several days thereafter, "but not for the whole two weeks." He was kept in a house with other hostages but said, "I couldn't speak to them." The Americans felt confident enough about their promised release to disagree with the students' claims that they occupied the embassy because it was a spy center and not a normal diplomatic mission entitled to immunity under international law.

Asked if he felt the embassy was full of spies, Maples, dressed in slacks and a leather jacket over a green shirt, replied, "No, I'm not under that impression."

Quarles, still wearing his uniform, said that while "Iranian people felt it was not an embassy," he "personally had no knowledge of spies."

Yet, he added, "under their ideology, I am sure they are right."

Asked what would happen if his captors had thought him a spy, Quarles said, "I'd probably be shot."

Maples said, "I don't think it was right," when asked if the students were justified in keeping the embassy staff hostage.

The first to learn of the impending release was Quarles, who said he was "given the impression last night" although he had not known "definitely until today."

He said he was "quite sure everything was being done in the States" to arrange the other hostages' release. "Just hold on a bit longer," he said. "I'm sure others will be released . . . Hang in there a few more days."

The three Americans seemed to be perhaps understandably uninformed about Iranian politics. Quarles arrived in Tehran Sept. 5, the others came in October.

Quarles said he had been treated, "pretty good" and "I made many good friends" with his captors.

"I learned a lot from what I read and saw," he added, "and was very saddened by some of the things going on during the shah."

Unlike the other two, Quarles said he had refused to sign a petition circulated by the students asking for the shah's return to Tehran to stand trial in return for their release.

"I didn't want to put my signature on something that might be derogatory to my government," he said.

Asked how he felt "about America and imperialism" he replied, "I think the American people have a lot to turn around and look at . . . There are always two sides and I saw the other side of the story and the other side of American imperialism."

Gross, dressed in a gray sweater over slacks, said, "I think it's awfully difficult to guess what he [the shah] did to the Iranian people and why he did it that way." She said she was presented with a book of photographs depicting women participating in the demonstrations last year and early this year that were instrumental in bringing down the shah.

Asked what she had missed most during her detention and what she would do first when she got home, she said, "The worst thing was not being able to get up and walk around when I wanted to. The first thing I'll do is call my family and tell them I'm still alive."