THIS WEEK'S symposium at the University of Maryland called "The Business of Singing: Survival for Singers" attracted about 200 people from widely diverse roles in opera, which is the most expensive of art forms and, some would say, the grandest. Here are glimpses at three of those persons, each with a different kind of job and at different stages of their careers.

W. Marsh Hudson, a baritone from Memphis, has decided at 36 that he is going to try a make-or-break shot at the big time. Betty Allen, 52, an American mezzo whose prestige has been greater than her fame, is winding down her career and giving more and more of her time to being executive director of the Harlem School of the Arts. And Matthew Epstein, 37, a power broker of the arts who recently quit as vice president and manager of that giant of the music world, Columbia Artists Management, is pursuing his goal of eventually running a major opera company.

Why did Hudson wait until he was 36 to attempt to spring out of an essentially obscure artistic existence in Tennessee where church music provided his principal livelihood? Robert Merrill is not the norm, of course, but he had already won his Met audition at 26.

"Well," replied Hudson, "I want a solo career. I just got it under my belt. What I want to do is the big leagues. Until now I just did not have the wherewithall to cut loose from Memphis."

A singer cannot keep his professional life separate. "One thing that held me up was that my first marriage fell through, and also I am just pulling out of my indebtedness. I have moved east to be closer to things, and will be at First Lutheran Church in Richmond. My career may not be so extensive as to be that of full-time opera singer, but I will try. In two years I'll know whether or not I'm a full-time opera singer."

Allen is rounding out 31 distinguished years as a mezzo.

She is proud of what she has done. A chance audition in 1951 led her to become a favorite of Leonard Bernstein, a man for whom she professes the greatest fondness. The audition was not with Bernstein, but with Charles Much, the conductor of the Boston Symphony. But Bernstein was there. "I remember that he turned to soprano Adele Addison and asked 'Do you think that she can sing the Jeremiah Symphony Bernstein's first symphony ?' Well, I had known his recording with Jennie Tourel for three years. And I apparently did well. Lenny became a sort of mentor."

Allen also speaks of what she has not done; she is philosophical. "For a singer who didn't make it big in opera, or make it big in Europe, I did pretty well. No one would contest that."

Matthew Epstein is a builder of singing careers, and if he has his way he will be head of a major opera company. But he gives himself a few years. "I need a little seasoning," he noted.

He is a dynamic, feisty person with big ideas and acute perceptions. Among the important singers he has helped form: mezzo Frederica Von Stade, soprano Catherine Malfitano, soprano Ruth Welting and baritone Richard Stilwell. Epstein practically grew up as a standee at the Met. His feelings about the voice are passionate.

In a telephone interview this week from Santa Fe, where he is singing in "Fledermaus," Stilwell made clear his opinion that he would not be where he is without Epstein.

"I give him credit for mounting my career. He is the man who got it started in the early '70s. And he thinks ahead. There was a time in the early '70s when we heard that Herbert Von Karajan was going to record Debussy's 'Pelleas and Melisande.' I had sung Pelleas so it made sense for me to audition for Karajan. That was 1973. When he was through he got up from the piano and said, 'You will be my Pelleas.' Imagine how that felt! But the years passed and Matthew and I thought he had forgotten. Then in 1978 Karajan decided to do it and I was called and we also made the recording." (It was also with Von Stade and is widely regarded as the finest version of the opera).

Asked about Epstein's ambition to become artistic director of a theater, Stilwell commented, "It's really just a matter of time. It's just when the right opportunity comes along. He would have one now except that there is just no reason why he should take over one of the lesser companies while he is waiting."