What in the world could the following three operas have in common?

* Massenet's "Le Cid," the heroic saga of the 11th-century Castilian warrior who fought Moors and Christians alike to conquer Valencia;

* Verdi's "Aroldo," an opera that started out as a drama about a German Protestant clergyman whose wife had committed adultery but had to be changed to the story of an English knight of the Crusades at the behest of the Italian church.

* Wagner's "Rienzi," an early work set in Rome of the Middle Ages where Rienzi was a tribune and a papal notary. The plot, which is beyond succinct summary, begins with an attempted kidnaping and ends five acts later with yet another burning of Rome.

Aside from the Middle Ages heroics they share, one thing these operas have in common is that you've almost certainly never seen any of them staged. Another is that so far as this country is concerned, they lay pretty well untouched in their respective operatic dustbins until they were brought out and performed in concert--all under conductor Eve Queler.

Sunday afternoon, Queler will bring "Rienzi" to the Kennedy Center in Opera Orchestra of New York's first appearance here.

But who, then, is Eve Queler?

She is an inordinately talented and enterprising musician, who has never seen "Rienzi" herself, who concluded about 20 years ago, when she was soon out of the Mannes School of Music, was playing the French horn and was coaching at rehearsals of the New York City Opera, that the inbred sexism at the major operatic institutions would preclude her ever rising to be a conductor.

So in 1968 she founded the Opera Orchestra, which grew gradually into a vehicle for concert versions of large-scale operas; now she conducts three operatic rarities a year in Carnegie Hall as part of a season largely sold out in advance. It has become an important addition to the New York opera scene, and Queler hopes that over several years she will develop a similar audience here.

She calls her work "exploratory rep." "This is not another chamber company," she explained in a recent interview. "In fact, we are not a company at all. We are an orchestra, with many of the same players who were with us in the beginning. And we engage singers to do individual operas. Some of what we do is because I want to do it. I most like to do masterpieces that you would never hear otherwise. 'Rienzi' is an opera I wanted to do.

"Then we also perform operas that the great singers want to sing, and that the major companies won't give them productions of. Next season, for instance, we're doing Berlioz's 'Benvenuto Cellini' for Nicholai Gedda. We've been working with him for years. My rule is never to go up against the big companies in an opera, though much of what we've done has been staged by them later."

You've never seen Eve Queler conduct at the Metropolitan. Nor has any other woman done so except Boston's Sarah Caldwell, who has been there briefly. "I'm not really sitting around waiting for the phone to ring," Queler noted.

And you probably weren't there the day she took the baton for the first and only time at the New York City Opera, a day that she continues to rue. "It was a Saturday matinee during Passover; would you believe it? And with no rehearsal I was to conduct 'The Marriage of Figaro.' I just knew that if it ever came it would be that way. The occasion was that Julius Rudel had a performance he couldn't conduct because he was away.

"But I had worked with the City Opera for years and on that production, so I simply walked out into the pit, introduced myself to the concertmaster, shook his hand and raised the baton for the overture."

Has she been asked back? "Well, you see, there has been a change of administration and, you see, Miss Sills hasn't invited me. Meanwhile she's been doing my operas one by one."

In the years she was seeking a career as a recital accompanist, she said, she encountered sexism from singers similar to what she has felt from the front office.

There are, for example, the critics. "Just last weekend I did the conducting at the New York concert tribute to Marian Anderson with Grace Bumbry and Shirley Verrett," she recalled. "There was a little roughness and one critic mentioned only that, while another wrote that I should be at the Met. You notice that one doesn't necessarily hear that negative kind of thing about a young man who is conducting."

When the recording of "Le Cid" with Placido Domingo came out, a critic for High Fidelity challenged her conducting on grounds that she considered sexist. "Placido had wanted to do an aria exactly as it was done on the Caruso record and I said no, because that is not what Massenet wrote, and that this would be the first recording and should be definitive. At the performance, no doubt under stress, he reverted to the Caruso version, and the record critic suggested that a male conductor might have had his way."

But for "Rienzi" she has made some cuts; otherwise it would go on for more than five hours. That length is fine for "Tristan," with which she scored a great triumph at the University of Maryland in 104-degree heat two years ago, but "Rienzi" isn't really a challenger to "Tristan's" supremacy. Instead, it is one of those works that many listeners know only by fragments--in this case the Overture, which pops up in the symphony hall from time to time, and Rienzi's noble prayer in the last act, which James King sang beautifully here last fall at the George London tribute concert. She has gotten "Rienzi" down to about 3 1/2 hours and thinks she has improved it.

One reason you don't encounter "Rienzi" much in opera houses is that it would cost about as much to stage as "Ai da." Here they will use Queler's 80-piece orchestra, nine soloists, a brass band and three choruses. One is the 120-member Choral Guild of Atlanta; there are 30 more singers from the Coast Guard Academy and there's the National Children's Choir from here. That's five busloads. She ordered food for this mob from a Washington caterer during the interview.

Wagner called "Rienzi, or the Last of the Tribunes" a "grand opera," based it on a Bulwer-Lytton novel, and finished it at 27, shooting for the rich and spectacular kind of hit that would score at the Paris Opera.

A quick listen to a recording--the only recording--showed that "Rienzi" is musically a considerably more exciting opera than those of Wagner's Parisian models, Spontini and Meyerbeer. Yet the first hint of "Tristan" is not there; it would come very soon in "The Flying Dutchman."

Queler, though, does not spend all her time fighting for obscure Wagner or Verdi. She spends a substantial amount of time conducting other orchestras, and there are moments when she does feel appreciated. "You know . . . at Carnegie Hall after Grace and Shirley had stopped singing, there were trees of flowers being brought out from all directions. The audience was standing and applauding. And suddenly someone was pushing flowers at me. I said 'me'? And there was this tremendous roar of applause. That made me feel very special."