It is too bad that the Endangered Species Act cannot protest the jazz vocalist, because the practitioners of this marvelous and moving art are truly a disappearing breed.

"The jazz singers who could go out there and just sing are not being developed any more, the environment is not here," said Betty Carter, one of the major jazz artists still singing today. "You can't find the young girls any more."

Carter said there is neither the time nor setting to develop jazz singers "who can work spontaneously for on-the-spot changes," not the demand from the record labels. "Most young singers today are trying to get a quick hit - and that means rock."

But luckily, some of the very best stylists jazz has ever produced are still singing and maintaining the tradition at a very high level.

The 25th Newport Jazz Festival, which opened Friday night, provided a showcase on consecutive nights for three of the very greatest - Sarah Vaughan, Ella Fitzgerald and Betty Carter - all at or near their prime.

For the vocal devotee, no greater treat could have been fashioned. Comparisons among the three would be unfair - the pleasure of each being unique.

But of the three, Fitzgerald, at 60, stands out as almost a miracle of nature, a singing wonder of the world. There are few singers in any area of music wo can boast such a consistency or breadth of achievement over such a long period.

Forty-four years after she won a singing contest at the Apollo Theater in Harlem, which whe entered on a dare, and was signed by the Chick Webb Band. Fitzgerald retains to an astonishing degree both the swinging agility and the girlish purity of her voice at its best.

The cliche about her is that she can sing the telephone directory and make it sound good.

Saturday night at Carnegie Hall she had it all and used it on a variety of material that ranged the gamut of jazz, blues and pop, including a swinging uptempo "I Cried for You" and a caressingly expressive "Angel Eyes" that brought the audience to its feet.

An encore of "How High the Moon." her hit of the 40s, allowed her to pull out every stop as she used her worldless vocalizing or scat singing, to mimic different instruments and quote snatches of other well-known songs.

For Fitzgerald, this was her second triumphant New York City appearance in as many months, and she seemed unusually relaxed and secure, compared to her often shy and ervous stage manner. She was accompanied by her long-time pianist, Tommy Flanagan, and his trio.

Sarah Vaughan, who opened the festival on Friday, has also paid her dues for over 35 years and is a diva equal to Ella in virtuosity but with a somewhat different voice. It has been described as "bituminous," referring to her ability to dig down and bring out rich sounds that seem buried deep in the earth.

Sarah first made her mark in the late 1940s in the bop clubs on New York's 52nd Street, where she was the jazz player's favorite vocalist but was also considered by critics to be somewhat eccentric and extreme in her styling. Today her voice seems less so, partly because it's more familiar and partly because she has tamed some of the more extreme swooping techniques that she used to lean on.

The overall instrumental effect is still there, but the top of her voice range is not quite as spectacular as it once was.

Also accompanied by a piano trio, Sarah did a luscious version of Tadd Dameron's "If You Could See Me Now," a sprightly Gershwin medley in which the shoo-be-doos in "Fascinating Rhythm" at one point resembled a Bach Fuque, and a sinuous "East of the Sum."

The encores included "Send In the Clowns" - a song that has been so overdone that a moratorium should be declared on its performance - and, finally, "Misty" and "Tenderly," which were two songs long associated with "The Divine Sarah."

Betty Carter's is a cooler and more abstract jazz singing style, sometimes resembling the way Miles Davis used to play trumpet, sometimes African tribal chants, and deriving much of its understated color and subtle expressiveness from the legendary Billie Holiday.

Her interpreations are also more radical - the version of "The Trolley Song" made you think you had never heard the song before - and also sound more contemporary and in tune with current jazz forms.

At the same time she has a more dramtic stage presence than either Fitzgerald or Vaughan, a flair for underliniing her vocals with a striking posture, and a defiant and liberated stance in both her public pronouncements and some of her songs that has won her a growing youthful audience.

Carter, at her Sunday night appearance in Philharmonic Hall, worked wonders with such unusual material as Cole Porter's "Most Gentlemen Don't Like Love, They Just Like To Kick It Around" (which was originally done by Mary Martin), getting Maximum meaning and humor out of its feminist sentiments.

After 30 years in the business, many of them in obscurity, Betty Carter, at 48, ironically may be reaching some kind of peak in her career as her reputation with new audiences continues to grow. She brought the audience to its feet in an explosive and spontaneous ovation that surprised Carter but no one else.

Following three nights of such superlative singing by these inimitable ladies, one is left with both a sense of exhilaration and a sad feeling that there probably won't be any more great jazz singers after they and some of their contemporaries are gone from the scene.