If Barry Manilow himself, the heart-throb of 12-year-old girls and Merv Griffin fans, were leading the PGA, it would be no greater surprise than the appearance of that long-and-lean Manilow look-alike - Rex Caldwell - atop the championship's leader board.

The 29-year-old struggling pro who says, "Sure, I'm a nobody," scalded Oakland Hills for a 66 in today's third round to take a two-shot lead over Ben Crenshaw and a four-stroke bulge over the dangerous trio of Tom Watson, Jerry Pate and David Graham.

If it had not been for a bogey on the 18th hole after a 45-minute rain delay, Caldwell might have tied the course record of 65 for one round, and the all-time three-round score in the PGA of 202.

"I'm not gonna lie to you," said Caldwell in his joyous hour after finishing his three days' work of 67-70-66. "I'm pretty sure I'm going to win.

"I may never have played a better round of golf in my life than I did today. I was just drawing out steel and blanking the flag.

"I was really amazed that I wasn't nervous," said the chap who is 47th on the money list and has never won tour event. "My swing is in the slot. I can't putt it any better and I never hit a nerve out there all day. I was just enjoying myself and living in the middle of the watering system (center of the fairway).

"Oh, sure, hell, I'll be nervous tomorrow. You can book it.That 10-year exemption for winning the PGA could take care of a lot of my business.

"Winning the PGA would be everything in life wrapped up in one for my wife and me. It would just about take care of my career," said Caldwell, who has never had a formal lesson and lived the hand-to-mouth rabbit life for years.

"I won't know until tomorrow how I'll play. Golf has no memory. No way I'll shoot 80. Well, I shouldn't say that...I might. I might bogey them all.

"But, you know," Caldwell said, his eyes narrowing as they usually do on the course, "I don't think so. I love the crowds, the kids running to see where I'm going to hit it. I'm a ham. I think it's going to bring out the best in me."

Caldwell revealed his mainspring, the key to his drive, earlier this week when, customary beer in hand, he said, "When I came out here, I had no golf game at all. I grew up collecting balls off the driving range to have enough money for greens fees the next day. I never broke par until I got out of college. I've never had a lesson. I started way behind.

"But," he said in dead earnest, "I've got more guts than the rest of these guys. I can will it in the hole. I want it more. When I finally learn how to hit it like the big boys..."

The wealthy golden boys on Caldwell's heels know his feelings and they don't care for it one bit. In his playboy years, they called him "Sexy Rexy," and now term him "Drain-O" for his uncanny knack on long putts.

When Caldwell says, "The difference between me and Barry Manilow is that I'm better looking," they cringe from the bottom of their elite souls.

Pate spoke for the clean-cut crew of Watson, Crenshaw and himself today - the model swingers who have seldom played their best under pressure.

"The only thing that could keep Caldwell from winning is the pressure," Pate said. "If he doesn't win, it'll be the pressure."

"If he can shoot 70 or even 71, he'll probably be all right. But I don't think he'll shoot it," Pate said. "He's got Watson and Crenshaw behind him and they can make it from anywhere, anytime. And my chances are pretty good, too."

"I feel good: I'm looking forward to tomorrow," said Watson, who played with Caldwell in a pairing that bristled with one-upmanship. As Watson passed Caldwell, he looked him in the eye and said, "See you tomorrow."

"I just need a good solid round tomorrow," Crenshaw said. That's not the comment he would make if Nicklaus or Watson were two up on him. "A lot of people have a choice to win. Anybody who is capable of a good round can get in it."

On tour, the younger players came in two basic groups. There are the clean livers like Watson, Crenshaw, John Mahaffey, Page and Hubert Green, who are all friends. Then there is a suddenly emerging group of naughty guys who have sowed their wild oats, talk a lot of jive and are starting to win the cash, too - Fuzzy Zpeller, Lon Hinkle and now Caldwell.

When Watson talks about pretournament preparation, he discusses his strategy meetings with Byron Nelson and Sam Snead. When Caldwell mentions his, he says, "I shot 68 in the last practice round and Fuzzy took all l my cash. He shot 64 and beat me more ways than you got toes."

At the ninth tee, Watson calmly watched Caldwell with a sort of bemused fascination, appropriate for the Stanford psychology major he is. Caldwell chatted with caddies, ladies in the crowd, marshals, then finally said to his caddie, "Let's put away the furniture (three-wood) today. I can reach it with a two-iron."

Caldwell blanked out the stick again, then pulled his walk-to-the-next-tee routine again as his 10-foot downhill birdie, his third straight and fourth in five holes, went in the front door.

After burning the lip at 10, then birdieing again at 11, Caldwell's bubble finally burst when he yanked a four-foot birdie putt at the par-five 12th.

Had he made it, he would only have needed to par into the clubhouse to break the course record. Instead, he ran off six pars in a row - bringing his streak to 29 holes without a bogey - before bogeying the final hole.

"I was sitting in the (TV) trailer," Caldwell said," and I almost missed my tee at the 18th. I had to come running over and put the peg right in the ground."

Caldwell says that if he had not made the final putt of the final tournament of the '75 season, to keep his tour card by $91, that he would be "punching keys in a supermarket." Now he knows that he has those three young paragons of golf virtue behind him - Crenshaw, Watson and Pate.

"I can't wait," Caldwell said. "Let's go ahead with the business."