John Mahaffey and Lee Trevino finally figured out the best way to play the Congressional Country Club course yesterday in the third round of the Kemper Open.

Forget the clubs. In today's showdown for a $72,000 first prize, just bring a rifle. (WDVM-TV-9, 4 p.m.).

"I call Mahaffey and myself the .30-.30s," said Trevino, one shot behind leader Mahaffey's three-under-par 207 total. "That's because we hit almost every shot as straight as a .30-.30 rifle bullet."

The blond, often-injured Mahaffey, the 150-pound short-knocking 32-year-old who always is straight as a string, turned in the only bogeyless round of this tournament yesterday. His three-birdie 67 equaled the low round of the day.

Of 390 rounds of golf here, only 31 have been under par 70, 10 of them yesterday -- and only Mahaffey has toured this 7,054-yard monster without a scar.

Behind the pleasant Mahaffey, who is as bland as vanilla ice cream on a bed of vanilla wafers, and Trevino, who shot 69, and is as spicy as tacos covered with tabasco, are three accomplished but unheralded players at one-under-par 209: Gil Morgan (70), Jack Newton (69) and Mike Morley (71).

Better known are Hale Irwin (70), who is tied with Jeff Mitchell at 210, and Craig Stadler (69), who is part of a trio at 211 with Mike Edwards and Jay Haas.

The day also brought three contenders to grief. Second-round leader J. C. Snead felt like impaling himself on the wild driver that led him to three bogeys on the first four holes and an ignominious 77 (214).

Even worse was poor Fred Marti, a 15-year tour struggler who was grouped with Trevino and Tom Watson in the day's glamor television trio. Marti started at par, but bogeyed the first seven holes and finished with 82. Fortunately, his hair was snow white before the round began.

Watson, the favorite of the modest crowd of 17,500, caught some of Marti's affliction, bogeying four of the first six holes. However, he finished at 73 -- 213 and at six shots behind still is on the edge of contention.

The moral of this first Congressional Kemper appears to be that neither long belting nor finesse touch around the greens is as important as old-fashioned accuracy.

Trevino and Mahaffey are tied for third place in greens hit in regulation (.717) in the PGA's new statistics, the best overall measure of straight hitting. Morgan, who battled a case of food poisoning all Friday night, is ninth (.696).

"It's going to be one rifleman chasing another out there on Sunday," said Trevino, who battled Mahaffey head to head in the 1979 Hope Classic and lost by a shot. "John doesn't take a back seat to anybody in staying down the watering system (the center of the fairway)."

"I was hitting it so crisp that for the first 14 holes there was only one hole where I had a chance to make anything worse than par," said Mahaffey, who is a cinch to have his fifth $100,000 year in money winnings.

"Maybe looking over your shoulder and seeing Lee hitting it 300 yards down the middle on every hole is bad for your nerves," said Mahaffey, "'cause I had to save pars on the 15th and 16th.

"I don't know if it was the tension of the moment or the heat -- Lord, it was a long, humid course today -- but I got a little rattled," said the Texan who has four tour wins, one the 1978 PGA.

Mahaffey, a cool and dogged customer, was so "rattled" that he lipped out a birdie putt at the 17th, then sank a gorgeous 25-foot double-breaking snake at the 465-yard 18th hole to take the lead alone.

The common denominator between Mahaffey and Trevino is more than their smallish size, and their perfectly repeating swings. Both have enormous will power.

They hardly seemed drained (despite protesting the opposite) after their rounds, while others seemed almost in a daze. (My mind's a blank. I'm worn out," said Newton, the hot-tempered Australian with talent but a penchant for blowing up after getting near the lead. "Everybody's sweating like a bloody pig out there."

Despite the lifelong similarity in the par-factory style of game that both Mahaffey and Trevino play, they actually are making their scores quite different now.

Mahaffey, one of the tour's shorter hitters, is ina machine-like groove. For instance, he had only one bogey in his 68 on Thursday. Trevino, by contrast, is in one of the best long-driving binges of his career.

"I sure am making enough birdies because my driving is setting up wedges to greens where I was hitting a five-iron here in practice rounds," said Trevino. "What's driving me crazy is all the careless bogeys I'm making from the center of the fairway. That's not me."

Trevino has 13 birdies here, but an uncharacteristic 11 bogeys.

"I haven't worked on my short game enough. Chips that I should put a yard from the hole I'm leaving 20 feet away. It's ridiculous. I don't like to chip, 'cause I don't believe in missing greens. Maybe that's my problem."

Actually, Trevino's only problem here has been deciding which clubs to carry. He is in love with a six-wood, but changes his mind each day on which club to leave in the locker.

"Friday, I left out the eight-iron and darn if I didn't need it three times," said Trevino. "Today, I left out the two-iron, and, sure enough, I needed it. Tomorrow, the four-wood goes.

"If they let me carry all the clubs I wanted, I'd need two caddies. Only Arnold Palmer owns more clubs than me. I'd carry six putters. I used the same (offset style) putter today as Thursday, but since then I've put in a new shaft and a new grip. Someday, I'm just gonna go nuts."

If golf to Trevino is a fierce form of joy, then two players here -- Mahaffey and Morgan -- approach this course with a touch of superstitious fear and bit of a desire for revenge.

Mahaffey's career is nine years of sustained excellence, interrupted by one 18-month side trip to perdition right in the middle. In 1975, he won $141,471. In '76, he played in the PGA here and hyperextended his left elbow hitting a three-iron on the 13th hole. In '77, he was 150th on the money list and almost disappeared.

That injury -- the first of three to his left arm or wrist -- still makes his psyche flinch. In a pro-am here on Wednesday, he deliberately drove away from the cursed spot where he was hurt.

Yesterday, his caddy told him on the 13th, "It's a three-iron." Mahaffey replied, "Give me the four-iron."

"Am I superstitious?" asked Mahaffey. "Yes, a little . . . or maybe you should say "careful." You have to remember, it was here that my career started going down hill and didn't stop until it was down the hill, around the corner and out of sight. There were plenty of times I never thought I'd get back."

If Mahaffey's memories are physically painful, then Morgan, the seldom-seen optometrist, has mental hurts. It was at Congressional in 1976 that he had a four-shot lead in the PGA after two rounds, then blew up with 75-75 for eighth place.

His mixture of Washington good and bad luck has struck again. Morgan had a severe case of food poisoning Friday night with "intestinal cramps and nausea." The doctor was worried enough to see a doctor yesterday morning.

"I have to watch out for the water here," Morgan joked. "I felt so lousy at the first tee that when I hit the first one straight, it surprised me. Actually, I felt better as the day went along. But I'm certainly tired now. I need to hold down some food for energy."

Morgan does not plan to eat at his motel again, nor will he practice putts in his room. "The slope of my room is too severe," said Morgan with the asperity of a man once-poisoned, twice bitter.

For those fond of handicapping final rounds, here are salient factors: Both Newton and Mitchell hit the ball so close to the hole yesterday that they were both moderately disgusted after shooting 69s. Trevino, Mahaffey and Morgan all mentioned casually that they had played nearly identical rounds all three days.

Only Morley, after his 71 on a course that was playing longer but mercifully slower after an early morning 42-minute rain delay, seemed honestly dubious about his position.

"I'm a streaky player," said the tall, enigmatic 33-year-old, a 12-season pro with one tour win in the Ed McMahon Open, plus a triumph in the 1978 North Dakota Open. "When I play bad, I really play bad . . . Usually, I've been playing early in the mornings (with the tail-enders) or not at all (after missing cuts). I'm not familiar with these late-afternoon rounds (with the leaders)."

While the likes of Mahaffey and Trevino were back on the tee for some sundown honing of their skills, Morley sought liquid refreshment.

"For the moment, my swing is in a groove," said the PGA's 113th-ranked money winner with a grin. "I'm just going to go home and let it rest."