John Mahaffey, considered a "has-been" at the age of 30, sank a 10-foot birdie putt on the second palyoff hole to defeat Tom Watson and Jerry Pate and win the 60th PGA Championship yesterday. The three had tied at 276, eight under par, after 72 holes.

Mahaffey got into the sudden death picture the hard way, by shooting five-under-par 66 in the final round while Pate had 68 and Watson took 73. Mahaffey was five strokes behind Watson going to the 10th hole despite being three under for the day's front nine.

But Watson faltered and a hot putting touch enabled Mahaffey to gain a share of the lead with him and Pate on No. 13, at eight under par. Mahaffey went to the front with an eight-foot birdie on 14 and made only one mistake the rest of the way, three-putting from 40 feet below the hole on 16 for a bogey.

This enabled Pate to take the lead briefly but Pate on 18 missed a four-foot putt that would have given the 1976 U.S. Open winner his second major title. Mahaffey and Watson, palying behind him, parred 18. All three players parrd the first extra hole, Watson scrambling slightly.

Pate was short of the green with his iron on the second playoff hole and chipped short. Watson and Mahaffey reached the green in two. Watson putted to within two feet of the pin, but neither Pate nor Watson got another chance as Mahaffey ended the tournament by sinking his sixth birdie putt of the day.

Mahaffey has been bothered by injuries and domestic problems in recent years. The 5-foot-9, 155-pounder from Houston had only one PGA Tour title to his credit, the 1973 Sahara Invitational. But his play was solid enough to earn more than $100,000 each season from 1973 through 1975, until he tailed off to 9,847 last year, ranking 150th on the tour.

This was the first three-way playoff in the history of the PGA, which was a match-play event through 1957. Since the change to medal play there had been three playoffs, Jerry Barber beating Don January in 1961, January downing Don Massengale in 1967 and last year, Lanny Wadkins stopping Gene Littler on the third extra hole. The 1961 and 1967 playoffs were at 18 holes.

Mahaffey posted 75-67-68-66 this week. Overall scoring was much lower than anticipated, due in large part to two days of rain that made Oakmont's treacherous greens so soft the field could go for the pins. When Johnny Miller won the 1973 Open here his four-day total was 279.

Mahaffey, Watson and Pate came within one shot of matching the PGA mark for most strokes under par under was set by Bobby Nichols in through 72 holes. The record of nine his 1964 win at Columbus, Ohio, where par was 70.

Mahaffey earned $50,000. Watson and Pate drew 25,000 each.

Settling for a share of second money had to be particularly disappointing to Watson, consistent leader the first three rounds with 67-69-67 and very much in control halfway through the final round. He eagled the ninth hole to go 11 under and rebuild the five stroke lead he had enjoyed over Pate at the start of play to four shots over Pate, five over Mahaffey and six over Tom Weiskopf.

Weiskopf faded to a fourth-place tie with Gil Morgan, who shot 67 yesterday, including a 245-yard hole in one; on the fly, with a one-iron on No. 8.

There was little to indicate during the early moments of the final round that Watson's big lead would fail to hold up. The "Golfer of the Year" in 1977, when he won the Masters and British Open, was even par over the first three holes, then moved tol under on the 561-yard, par-5 fifth hole by reaching the green in two and two-putting from 20 feet for a birdie.

Watson was six strokes ahead of Pate at that point, driving long and accurately, maintaining his excellent touch on and around Oakmont's soft greens.

He encountered trap trouble on the next two les, however, and settled for bogeys. Still, his lead looked safe, as no one had started to make a strong run at the leader. Then, when Watson struck his best shot of the day on the nonth fairway - a four-wood on the par-5 hole to within two feet of the pin - many fans in the large gallery were ready to concede to 28-year-old Stanford graduate his fourth major titie.

That four-wood was a thing of beauty, on the flag all the way, assuring Watson an eagle and rebuilding his lead to four strokes over Pate, five over Mahaffey and six over Weiskopf.

But Wastson quckly gave back the two strokes he had picked up onno. 9. His drive on 10, where the fairway slopes downhill much of the way, was straight and true. His iron was heavy, however. It caught a trap at the front left-of the green, and his blast out of the trap left him some 30 feet from the pin.

Watson's putting at Oakmonhad been exceptional through 63 holes, as steady a stroke as anyone would hope to see in a top tournament. And his first putt on No. 10 was good, to within two feet.

But Mahaffey picked this green to sink a 15-foot putt for a second consecutive birdie. Watson failed to tap in his two-footer, causing a doublbogey, and the chase was on.

Pate and Mahaffey both caught the faltering front-runner on the 13th hole, all at eigh under.

Mahaffey's brilliant putting streak gave him the lead on 14 and 15. Pate brielfy went to the lead at 17, at nine under, but bogeyed 18 by three-putting from inside 20 feet.

This oped the door for Watson. He appeared to be out of contention after his iron landed in the right rough on the par-3 No. 16. He chipped past, missed a putt coming back and was two strokes behind Pate going to the 17th tee. Pate had just birdied 17.

But Watson refused to quit, now that he had helped knock myself out of the top spot. He, too, birdied 17 and parred the final hole. His approach to the 18th was excellent, hitting within eight feet of the stick, only to roll back toward the gringe, leaving him 18 feet away, from where he parred.

Mahaffey three-puttee 16 to surrender the one-stroke edge he enjoyed overpate there but held steady through 17 and 18 with pars to join Watson and Pate in the three-way playoff, rare in the history of a major golf event.