Only a few weeks ago, racing fans were deploring the dismal quality of America's 3-year-olds. But no such complaints are being heard now, for Saturday's Belmont Stakes has developed into an extraordinarily competitive and fascinating event.

Rarely has this historic race attracted such a depth of talent. Linkage, Aloma's Ruler and Conquistador Cielo are so good that they have relegated the stretch-running winner of the Kentucky Derby, Gato del Sol, to fourth choice in the morning line.

This confrontation is especially intriguing because the three favorites are all speed horses who have gone to the lead in mmost of their races. So the outcome of the 114th Belmont (WDVM-TV-9, 5:38 post time) will be greatly determined by the strategies of the jockeys. It will also be influenced by another, less tangible factor: the genes that determine thoroughbreds' ability to go 1 1/2 miles.

The pivotal horse in the Belmont field is Conquistador Cielo, who was not even listed as a possible entrant a week ago but then won Monday's one-mile Metropolitan Handicap in track-record time. On the strength of that performance, he will probably be the favorite Saturday.

"He is the quickest horse in the race," his Hall of Fame trainer, Woody Stephens, said, and nobody is arguing. In fact, the colt is so quick and so sharp now that jockey Laffit Pincay may have trouble controlling his speed in the initial stages of the Belmont.

(Eddie Maple has been Conquistador's rider, but Maple suffered a fractured rib and kidney injury in a spill today in the sixth race at Belmont. After the ninth race, Angel Santiago, scheduled to ride Anemal Saturday, fell off winner Billowy and later complained of lower-back pains. He will miss the Belmont; no replacement was announced.)

Conquistador Cielo's early fractions in the Belmont may be critical. "If any of the good horses gets away with a half-mile in :48 or :49, he'll be long gone," said Angel Cordero Jr., who rides Royal Roberto. "But if they go the three-quarters faster than 1:11, I don't think the speed horses will stay."

Behind Conquistador Cielo, 16-year-old Jack Kaenel and 50-year-old Bill Shoemaker will have tough decisions to make, trying to decide when they will make their moves. They can't let the leader steal off to a big early advantage, but they know that trying to run head and head with Conquistador Cielo may amount to suicide.

"It's not an ideal situation for us," conceded Butch Lenzini, trainer of Aloma's Ruler, who had an easier time of it when he led all the way to win the Preakness. If his jockey, Kaenel, does make the first move to challenge Conquistador Cielo, Shoemaker will inherit an ideal position, sitting third on Linkage behind the two dueling leaders.

When the horses turn into the stretch and find themselves going farther than they have ever run before, even perfect racing luck won't help them much if their genes do not cooperate. This is a crucial consideration in the case of Conquistador Cielo, whose bloodlines suggest that a mile is his optimal distance. If a son of Mr. Prospector wins at 1 1/2 miles, the pedigree experts will have to reprogram their computers.

There are a few questions about the stamina of Aloma's Ruler, fewer about Linkage, whose sire and dam have both produced successful distance runners. But the most stoutly bred horses in the field are Gato del Sol and Royal Roberto. They will have to come from more than 20 lengths behind, but there is no doubt about their ability to handle the distance.

The other entrants in the 11-horse field are Anemal, High Ascent, Cut Away, Estoril, Illuminate and Lejoli. None of them can be considered a serious contender.

If this Belmont Stakes were not complex enough already, heavy rains were being forecast for Saturday. A sloppy track would probably hurt Gato del Sol and Royal Roberto, whose races in the mud have been bad ones. But the trainers of the favorites were not fazed by the prospect, even though their horses have not run on an off-track before.

"Linkage doesn't care much what the condition is," Henry Clark said. "He had a good workout at Pimlico on a wet track."

"I don't care what the condition of the track is," Lenzini said. "I would think that if the track is sloppy, it would be to the advantage of the three speed horses."