Tom Watson beat back an early challenge from Tom Weiskopf and eventually lengthened his lead in the 60th PGA championship to five strokes yesterday by carding four-under-par 67 in the third round of the season's last major golf event.

The third straight sub-par efort enabled Watson to increase his advantage over Weiskopf. Johnny Miller and Joe Inman, who were among his closest pursuers at the start of the round. It also kept at bay strong rallies lodged by Jerry Pate, John Mahaffey and Craig Stadler.

Pate holds second place, one shot ahead of Weiskopf and Inman, with Mahaffey and Stadler and Lee Trevino the only other players under par.

A light rain fell throughout the afternoon. This was responsible for the lowest scoring of the tournament as players were able to challenge the pins on soft greens.

Watson has mastered Oakmont in 67-69-67 for 203. 10 under par. Her started slowly yesterday, bogeying two of the first three holes. But his booming drives and deft touch around and on the greens netted six birdies, including 12, 13 and 14.

Pate, Mahaffey and Stadler moved into contending positions by joining Gil Morgan (66) and Kermit Zarley (67) among the day's hotshots.

Pate, the 1976 U.S. Open champion streaked from one-under to five-under for the tournament on the last seven holes. He finished with 66. Mahaffey sustained his strong play from Friday's back nine, when he had 31, with 68 yesterday. Stadler's 67 took him from two-over to two-under for 54 holes.

Watson started the round four strokes ahead of Weiskopf. Crenshaw and Inman and five in front of Miller. He bogeyed the first and the third holes, however, in the slippery going and, before sinking a 15-foot birdie putt no No. 4, was tied for first place by Weiskopf.

Weiskopf recovered from a bogey on No. 1 to birdie the second, then hit the most spectacular shot of the day when he sank his iron shot from the fairway for an eagle-2 on the fifth hole. That put Weiskopf and Watson at four under.

Watson was equal to the early challenge. Less than three minutes after Weiskopf registered his eagle Watson gained his first birdie. The 28-year-old graduate of Standford University than curled home a tricky 12-foot putt on No. 5 to go back to six under. He completed the front nine with pars on each hole. His drives were extra long, his overall play exceptionally solid.

Weiskopf was paired with Crenshaw and Bob Zender. Crenshaw slipped to a three-over 39 going out as his excellent putting stroke failed on several attempts inside eight feet. Zender was unable to rally from his starting point of even par.

Miller and Inman were in the threesome with Watson. Inman does not have the power to stay with these two off the tee but he held up nicely nonetheless by carding even par at the turn. Miller, meanwhile, went from one under to three under to even and back to one under after nine.

The eighth hole, apar-3, 255-yard torture test, again proved critical. Weiskopf and Miller suffered double bogeyed while Watson parred. This hole has been Oakmont's toughest this week and Weiskopf and Miller made it look impossible as they sprayed their tee shots far off the desired track. Both players bonced back with birdies on the par-5 ninth hole but when Weiskopf continued his up-and-down doings with a bogey on 10, while Watson birdied, the scoreboard showed Watson enjoying his biggest margin of the tournament, five strokes, over his closest pursuers.

The best score posted early was by Dr. Gil Morgan, the nonpracticing optometrist from Wewoka Okla. He fired a bogey-free 66 that featured four birdies on the front nine. It was the lowest round of the tournament to that point.

Morgan sank putts of 10, eight and 25 feet for birdies through the first six holes. He chipped to within four inches for another birdie on No. 9, then curled a 15-footer into the cup on 15 to cap his sparkling effort. Johnny Miller's 63 in the last round of the U.S. Open here in 1973 is the course record.

"You think about records like that occassionally," Morgan acknowledged "I was far enough under at nine to think about it, certainly, but I parred the next five."

Morgan's five-under figure followed 76 and 71 the first two rounds. He started out the day 11 strokes behind which was 148. Now, suddenly, he was thinking about the possibility of winning.

"If they come back a little bit today, and I have a round tomorrow like I had today, I feel I'll have a chance," he said. "My trouble was I was four over after the first three holes on Thursday."