HAGLER'S TRIANGLE COVERS ALL ANGLES

By PHIL BERGER, SPECIAL TO THE NEW YORK TIMES

Published: April 3, 1987

LEAD: When boxing champions step into the gym, more often than not they are preceded by large entourages.

When boxing champions step into the gym, more often than not they are preceded by large entourages.

Not Marvelous Marvin Hagler, the middleweight champion who on Monday will defend his title here against Sugar Ray Leonard.

At Palm Springs, Calif., where he trained evenings in a large white tent, Hagler would materialize suddenly with just two men at his side.

One hour later, as he headed back to his room amid the insistent one-note of crickets and barely stirring eucalyptus trees and Mexican fan palms, the same two men would be with him.

''The unbreakable triangle'' is how the middleweight champion and those men of his - Pat and Guerino (Goody) Petronelli, his co-managers - refer to a relationship that has survived a seeming eternity in a difficult business.

''It's taken all of us to get there,'' said Hagler. ''The triangle - we stick together.'' The Whole Show

The relationship started in 1970 when Hagler, a 16-year-old high school dropout, walked into a gym in Brockton, Mass., run by the Petronellis to become a fighter. Now, going on 17 years later, he is still with them. The Petronellis and Hagler - that is the show.

''It is the strongest and closest relationship that I've seen between a fighter and his boxing people,'' said Bob Arum, Hagler's promoter.

Though the Petronellis have been in boxing for many years, tracking dates from their distant past is no easy job. The subject of their birth dates, for instance, is an occasion for winks and wisecracks and a somewhat reluctant admission, eventually, to their being in their ''late fifties.'' Goody is the tall, lean Petronelli, the one who is still fit enough to match strides with Hagler on the fighter's 6- to 10-mile training runs.

Pat, who admits to being two years old than his brother, is the more rumpled-looking and outgoing character. His idea of roadwork is to drive his Cadillac through traffic.

Goody fought as a professional (he says his record was 23-2-1) until a broken hand ended his career. At that point, he enlisted in the Navy and was mustered out in 1969 with 20 years of service. Double Shift

Pat was an amateur fighter but, by his own admission, was not enamored of training. While Goody was in the Navy, Pat worked in the morning for their father's construction company and followed that with a late-afternoon shift as a carpenter at a shipyard in Quincy, Mass.

In 1965, Pat opened a gym and planned to establish a larger one when his brother got out of the Navy. Originally, they planned to be partners in the gym with Rocky Marciano, the former heavyweight champion who was also from Brockton.

In August 1969, just discharged from the Navy, Goody was driving his convertible through North Dakota on his way back to Brockton when he heard a report on the radio that Marciano had been killed in an airplane crash. The brothers opened up their gym on their own and, within a year, Hagler walked through the door.

What Hagler began with the Petronellis, the gym was strictly a sideline - operating evenings after the Petronellis and Hagler (who worked for them) made a day's wages with the brothers' construction company. 'Rough, Tough Street Fighter'

''Marvin had hair then and had moved up from Newark with his mother and the rest of the family,'' Pat Petronelli said. ''He was a very quiet kid. A rough, tough street fighter who was very strong and had a lot of heart.''

During the day, Hagler would earn $3 an hour digging ditches, mixing cement and passing boards. In the evenings, he labored under the Petronellis in the ring.

''He impressed me,'' Pat Petronelli said. ''He had something others don't have. He had reflexes and a sense of when he could hurt someone. And he could't get enough hours in the gym. The first one in, the last one out.''

As co-managers, the Petronellis have separate and distinct responsibilities with Hagler. Pat handles the business matters and has been more involved with Hagler's personal life than Goody has, He is, for instance, the godfather of Hagler's 6-year-old daughter, Charelle. For his part, Goody handles the fighter's training.

'The unique thing is that neither guy alone could fulfill Marvin's needs,'' Arum said. ''I don't think Marvin would have that much confidence in Pat as a trainer, and he doesn't discuss personal and business matters with Goody. Ever. Pat's role is the father figure. Pat deals not only with Marvin's business situation, but Marvin goes to Pat with personal things. So there is a strong bond there.''

During his career, Hagler has relied on Goody's boxing advice to a degree not common for fighters who have reached the heights he has. 'A Collective Kind of Fight'

''Marvin works out a game plan with Goody and will follow it, in training and in a fight,'' Arum said. ''He's not particularly innovative on his own. I can't believe Leonard would do the same thing. Leonard might discuss a game plan with Angelo Dundee and the others, but once the fight started, he'd wing it on his own. Marvin is the only really great fighter I know that will get a game plan and stick to it and only change when he and Goody decide to change. He fights a collective kind of fight, which is rare for a great fighter.''

When Hagler fought Roberto Duran in November 1983, he was criticized for sticking to the conservative game plan, strategy that Pat now concedes was probably ill-advised.

Over the years, though, the Petronellis' counsel has worked to their fighter's advantage, and Hagler himself is quick to credit them.

''In the beginning,'' he said, ''they never took a dime of my purses. They told me, 'One day, we'll all make money.' And now we are. And it's good to see them get the respect they deserve.''