For any good 3-year-old racehorse, this is the season of truth.

A thoroughbred can go through his 2-year-old campaign and the Triple Crown series insulated from open competition, running strictly against members of his own generation. He may be able to win consistently and look good because his contemporaries are so bad.

Only when he ventures into competition against his elders can a 3-year-old's ability be measured and put into perspective. Saturday, Pleasant Colony will be subjected to that test.

The best 3-year-old of 1981 will take on nine older rivals in the $200,000 Woodward Stakes, the first leg of Belmont Park's fall championship series that is designed to identify the best horse in America.

Trainer Johnny Campo, who normally exudes unshakable confidence, recognizes that this is a critical moment in his colt's career. "This race will prove how good he is," Campo said. "There's nothing you can do to take away from what he did this year, but the Woodward and the other races will prove if we can call him a great horse."

It is likely the answer will be negative. Pleasant Colony's victories in the Kentucky Derby and the Preakness came at the expense of a demonstrably weak group of 3-year-olds. In his victories as well as his defeats, Pleasant Colony revealed an utter lack of versatility. The only way he can run is to start slow and try to seize command of a race with a big move on the turn.

Pleasant Colony's chances to win the Woodward were boosted when the owner of Fappiano, the best older horse in the country, decided to pass this race and await the richer Marlboro Cup two weeks hence. But Pleasant Colony must still face a tough, seasoned group of horses.

The most reliable of them is Amber Pass, a 4-year-old who is based at Pimlico and owned by a physician from Frederick, Md. Amber Pass had always shown potential, but he has blossomed this summer and won the Monmouth Handicap impressively in his last start. He is the legitimate horse to beat Saturday, but there is one small knock against him. Amber Pass has been running well in handicaps while carrying relatively modest weight, but now he has to tote 126 pounds. This is the first time he has been forced to concede weight to good horses.

Relaxing, probably the best mare in America, devastated rivals of both sexes during the winter and spring; Maryland race-goers will remember her smashing victory in the John Campbell Handicap at Bowie. After a five-month layoff, Relaxing won a recent stake at Delaware Park in a fashion that suggested she is back in peak condition.

Herb Water is a horse that only a speed handicapper could love. The colt has never shown he belongs in top-class company, and he has already lost to most of his rivals in the Woodward. But last week he ran the best race of his career, losing by less than a length to Fappiano and missing Belmont's track record for a mile by just two-fifths of a second. That performance may have been a one-shot fluke -- but maybe not.

The others in the field are Temperence Hill, Fio Rito, Winter's Tale, Peat Moss, Joanie's Chief and Ring of Light. All of them come into the Woodward with very reputable credentials and a chance to win based on their best prior performances. If Pleasant Colony can prevail in this company, nobody will ever be able to ask again, "Whom has he beaten?"