Tom Watson assumed the lead after three rounds of the British Open today the same way he won last year, when somebody else faltered down the stretch. This time he was in a mid-sip of water during a press conference when Craig Stadler made double bogey on the final hole.

It was an odd sort of round. Stadler, with a one-over-par 72, did not play poorly in losing the lead he held the first two days and Watson (70) did not play brilliantly in gaining it, by a stroke at eight-under 205. Lee Trevino (73--208) slipped slightly, Ray Floyd (69--207) and David Graham (67--207) charged to move into contention. And Nick Faldo (71--207) kept himself composed and Britain's hopes for a native winner soaring.

The sad reality of the day was that the worst damage to Royal Birkdale happened about nine hours before a golfer laid a club to it. Sometime after 11 p.m. Friday, vandals all but wrecked the toughest hole on the course, the sixth, ripping more than a dozen patches, some more than a foot deep, and painting messages in support of convicted murderer Dennis Kelly on the green.

The sixth is the farthest from the clubhouse and nearest the beach area here. Four boys spotted the damage and quickly reported it, officials said, and repair work started about midnight. The round was delayed 20 minutes and the tee moved forward by 40 yards, so the players could carry a fairway bunker with a driver.

That left a reasonably short second shot to the par-4 green. Once on, players were given putting relief in certain situations. Three of the leaders, Floyd, Graham and Trevino, made birdie and nobody within five shots of Watson had worse than par.

Extra security, with dogs, and about two dozen unarmed military personnel from a nearby barracks, patrolled the grounds tonight.

With the exception of Jack Nicklaus and Seve Ballesteros, the players nearly everyone expected to contend for the title the final round will. Ballesteros is at two under, but at least a shot behind a dozen players; Nicklaus failed to break par for the third straight day and trails Arnold Palmer (68) by one.

The final-round scenario is a beaut. In contention are Watson, with four British Open titles; former Masters champion Stadler; Floyd, the reigning PGA champion; Graham, who shot a final-round 66 to win the 1981 U.S. Open; former Masters winner Fuzzy Zoeller (67--209); two-time U.S. Open titlist Hale Irwin (72--209); Andy Bean (70--209), who has never won a major, and Faldo, who has won three European championships recently.

Irwin would be a stroke closer had he not whiffed about a four-inch tap-in on the 14th hole. Backhanded, he stabbed at the ball and, inexplicably, pulled the putter completely over the ball. Instantly, he knew it counted as a shot and was visably shaken. Then he executed what he had in mind all along, but twice fumbled the ball plucking it from the hole.

"I have mixed emotions (about going into the final round ahead)," said Watson, who was caught from behind and passed in the U.S. and Western opens within the last month. "That's the place you want to be, but I need to straighten out the drive a bit.

"A lot, actually."

Watson used his driver four times today and hit it miserably three of them. He suffered a double-bogey 6 on the first hole after driving into an umplayable situation. He twice saved pars after awful drives, and even birdied the 17th. On the final hole, he got up and down not far from the clubhouse patio.

He was miffed for other reasons. Paired with Faldo, he was victim of some rude rooting. A man cheered a missed birdie putt at the second hole, and the wonder is that he stayed alive during the bolt of anger Watson's eyes shot at him. Another fan booed when Watson made a tricky putt at No. 7.

"Wouldn't you be cross?" Watson said, adding that he glared so long and hard during the second-hole incident, "because I just didn't want him to get away with it."

Watson's wish Sunday is "a round like (Johnny) Miller had." That would be the 66 that pulled Miller ahead of Nicklaus and teen-ager Ballesteros in the '76 Open at Birkdale. He thought 69 might be enough, even 71.

Much of what happens depends on the weather.

"If it's calm," said Stadler, "my chances are very, very good. If the wind blows, I'll have to work a little harder."

He misjudged the wind on the final two holes today. He saved par at 17. But paid dearly for the foul ball a hole later. After pitching back into the fairway, Stadler nailed a three-iron to the par-4 hole into a green-side bunker. A blast and two putts from 30 feet ended a one-over 72.

Playing partner Trevino escaped a wide tee ball at 18 with a 20-foot bogey putt "that felt like a birdie. I was starin' six right in the face,"

For maybe 30 seconds, Trevino had a share of Stadler's lead. On the second hole, he sank an 18-footer for birdie. Then Stadler eased in a seven-footer. He coaxed in another birdie a hole later for a four-shot lead on the field.

But bogeys at the fourth and ninth holes let Watson and the rest stay close. Stadler also saved par several times with putts the length of a small walrus.

"That drive at 18 felt felt pretty good when I hit it," he said of that final-hole disaster. "But it looked like a pigeon flying off. I was pleased when I hit it, but it nearly ended up out of bounds. I got fooled by the wind."

Trevino was everything but ordinary.

"Some great shots and some bad shots, nothing in between," he said. "But more bad than good. I feel good about tomorrow. I'm hittin' it well off the tee and puttin' well. It'll be extremely difficult for players in the last group (Watson and Stadler), because of 49,000 photographers, 25,000 carts and people being allowed onto the fairway to watch putts."

Trevino and Faldo are paired in the third-from-last group, with Floyd and Graham immediately ahead. If Faldo stays hot, the tradition of British crowd courtesy could get smashed. Some of these folks sound like football fans.

"Come on, Nicky, you bulldog," a deep-voiced man yelled as Faldo was approaching the 18th green after a chip to nine feet. Another voice screamed: "Come on Nick, baby."

Faldo missed that par putt, but was not annoyed for long.

"Just incredible," he said of his support. "It bubbles over into everything. When we slip into town for whatever, people want to talk. They're all there rooting. I just do the golf."

Also, the 18th hole is designed to push a man's courage to the limit. With nothing more than get-in-contention pressure on them today, Stadler doubled the hole and Faldo, Floyd and Trevino suffered bogey. Nobody birdied it all day.