The middleweight champion, Marvin Hagler, and the outgrown welterweight with the dreaded right hand, Tommy Hearns, have signed to do combat for Hagler's title May 24. With a certain license, it may be said that the third man in the ring will be Sugar Ray Leonard.

No matter where Leonard may be on the night of May 24, his image will hover over the scene. He will be as fixed in the minds of Hagler and Hearns, and as visible, as the gloves tied to their fists.

Each knows that the next step is to meet Leonard, who makes multimillionaires of those he fights. As the late Willie Sutton would have remarked, Leonard is where the money is. To fight boxing's Mr. Box Office is now likened to a raid on the bank, a heist guaranteed worth millions, win or lose.

Hagler-Hearns, whether in Nevada or Hearns' home Detroit area, is an inviting fight in its own right. The rugged champion, his shaven head giving the appearance of more ferocity to his muscular body, is undefeated in 29 straight fights. He'll be defending his title against the Hit Man, Hearns, who will stand taller, will outreach him and is content to give away five pounds for a shot at the 160-pound title. Hearns could be the betting favorite. It is remembered that it took Leonard to stop Hearns (34-1).

There was almost a Leonard-Hagler middleweight title fight on the late May date given to Hearns. But Mike Trainer, Leonard's attorney, broke off those negotiations because the Hagler people took a very presumptuous attitude: they wanted as much money as Leonard, which in Trainer's view is not an entitlement of any Leonard opponent.

Trainer has put it simply: "Nobody should get as much money as Leonard. I assume Hagler's people should see that, too." As if in further contempt of Hagler's drawing power, Trainer said, "The biggest purse he ever got was $1 million, and the promoter lost money on that fight."

In prize ring lore, the middleweight title was always vastly more valuable than the welterweight title and denoted bigger purses. But that was before the charismatic showman killer, Leonard, came along to convert his title into a precious asset, more marketable in recent years than even the aggrandized heavyweight crown.

There were gasps when Larry Holmes and Gerry Cooney were guaranteed $10 million each for their upcoming heavyweight fight. Yet Leonard netted more than that for his 14-round technical knockout of Hearns last September (he was guaranteed $8 million). Hearns' guarantee was $5 million.

Leonard's worth at the box office also was measured this February when Wilfred Benitez defended his super-welterweight title against Roberto Duran, Benitez getting a paltry $1.1 million guarantee. Duran's allotment was $700,000. For Duran, the difference was no Sugar Ray Leonard. Twice against Leonard, Duran walked away with $5 million.

As proof of how the purses diminish if Leonard is not one of the parties to the contest, Hagler-Hearns is another case in point. On May 24, with Hagler risking his title against the big-hitting Hearns, the word from promoter Bob Arum is that Hagler is guaranteed "slightly over $2 million and Hearns $1.25 million."

In good time, Leonard expects to take on either Hagler or Hearns for the middleweight title. His trainer, Angelo Dundee, says Leonard, who trained down to 147 for his title defense against Hearns, could easily move to 154 or 155 and fight as a middleweight, five pounds below the limit.

Leonard could be more interested in fighting Hagler than Hearns, remembering those first 13 rounds when he was behind on the scorecards before unloading the salvo that salvaged his title in the 14th, when the referee took mercy on Hearns. The Leonard people secretly could be rooting for Hagler vis-a-vis Hearns. They are convinced Hearns will never move down to the 147-pound limit again. But when the money is right they would not duck a fight with Hearns, they indicate.

A virtue of fighting Leonard is that opponents get an uncomplicated deal. With the fight game's best drawing card in hand, Trainer deals directly with the networks or closed-circuit people, eliminating the promoter, or middleman, to the benefit of the entire purse. In those terms he has revolutionized the business. No promoters. Just the fighters and the TV people splitting the swag.

In contrast, Hagler and Hearns are fighting under the aegis of promoter Arum, who will count himself in for a share of the loot, his privilege. But Leonard's people do not like to do business that way.

In his last appearance, Hagler scored a 66-second knockout over one Caveman Lee, of small repute. The last time he lost was in 1976 to Willie Monroe, but Hagler does not knock everybody out. In 1980, one who went 10 rounds with Hagler was Marcos Geraldo, lately stopped by Hearns in quick order.

Hearns will be recognized as the puncher in the Hagler fight, despite giving away pounds. His problem might well be that he will be fighting a right-hander turned effective southpaw. Hagler, if not a stunning knockout artist like Hearns, is nevertheless a bruising battler and a certified champion. A problem for Hagler is that he is not the kind to make Hearns miss, as Leonard did.

There it is again, that name: Leonard. In a special sense, those punches being thrown May 24 will be aimed his way. That's all right with Sugar Ray, who will be lying in wait for the day. At his price.