Who swims 2.4 miles, bikes 112 miles, then runs a marathon?

To many sports fans the name Dave Scott means nothing - yet he is perhaps the best-conditioned athlete in the world. Mr. Scott competes in the triathlon, a new and growing sport that usually involves swimming, biking, and running, but sometimes includes kayaking, cross-country skiing, or canoeing. The competition is compacted into a single day, with no break in the action.

The ultimate test, and the original triathlon, is the Ironman World Championship in Hawaii. Scott, from northern California, has twice won this event, which combines a 2.4-mile ocean swim with a 112-mile bike race and a marathon run - 26 miles, 385 yards. His best time of 9 hours, 8 minutes stands as a record.

John Collins actually ''invented'' the triathlon five years ago. He was in a Hawaii restaurant with friends, all of whom were bragging that they were in the best physical shape. Since the group consisted of swimmers, cyclists, and runners, he proposed a three-phase race. About 15 people ran in the first two Ironman triathlons. Sports Illustrated magazine (SI) carried a lengthy feature story about the second one. Now the sport has its own magazine.

''The sport was kind of created by the media, when SI covered the race in 1979,'' says Harald Johnson, cofounder of Triathlon magazine. ''The sport is accelerating. Some people may think it's a fad, but we tend to think of it as a new evolution in fitness. Multisport activities are a new challenge for people who get tired of one thing. Put three sports like swimming, biking, and running together and you have used most all the muscles in the body. It's a positive new concept - total fitness.''

This year more than 100,000 people are expected to enter more than 300 triathlons in the United States. Of course, not all of the races will be as long as the Ironman. Most will likely be shorter.

''A lot of the races are designed for the working man or woman to compete in, '' Mr. Johnson explains. ''There are short-course races which last around two hours, and middle-distance races which average two to four hours for the top finishers. A businessman can train an hour every morning and a little more on weekends to get in shape to run these middle-distance races.''

The sport now has sanctioned, professional races. The United States Triathlon Series (USTS) will have 12 races this year. All consist of a 11/4-mile swim, 25 -mile bike route, and a 9-mile run.

There is quite a bit of triathlon activity in California, where the public's attitude is very supportive, Johnson says. ''In other areas they may think you're crazy.Also the weather is perfect for training.''

Living off earnings as a professional triathlete is not easy. First-place finishers in the USTS races receive only $1,000, and the Ironman winners get nothing except good press coverage. The handful of athletes who support themselves in this sport do so primarily through endorsements and public appearances. Scott has sponsors, gives talks, and advertises products so he can afford to train eight hours a day to stay in top shape.

He has an unusual philosophy on the sport. ''I think most people probably put too many limits on themselves. A lot of people limit their potential with a negative statement rather than expanding it by a positive affirmation,'' Scott says. ''When you set your goals, make sure they end on an optimistic note. Extend yourself, keep going faster, harder.''

All triathlons, not just the long ones, require serious training because of the smooth transition needed from one activity to another. One must step out of the water onto a bike, and that means using muscles in different ways.

The most difficult transition may be bike-to-run. Johnson, who ran the Ironman in 12 hours, 24 minutes last year, recalled his feelings in making the switch. ''There I was after biking 112 miles, and I had a 26-mile marathon waiting for me. Besides the fact that my legs were tuned to biking, mentally the thought of running that far was incredible. You just have to hang tough and go for it.''

The biggest recent boost to triathlons may have come through ABC Sports television coverage of the last two Ironman Championships. In the 1981 race, millions saw Julie Moss crawl the last yards trying to win the women's title. She was overtaken just before the finish line.

Current Ironwoman Julie Leach shares the view of many participants about the importance of the mental aspects of competing.''A lot of it is just imagining what you want to do and plugging in positive reminders.''