Moments after the Miami Dolphins had won their conference championship and advanced to the Super Bowl, Gary Austin's Sports Book started taking wagers on their game with the Redskins. Testing public sentiment, the bookmaking establishment made Miami a 2 1/2-point favorite.

"People were standing in line, just waiting to bet the Dolphins," said manager Tom Stadel. "We couldn't stop the action on them. After five minutes, they'd bet so much that we had to raise the spread to 3 points."

That was the opening flurry in a week of action that will make Super Bowl XVII the biggest betting event of the year in America. Some $30 million will be wagered on the game legally in Nevada; countless tens of millions of dollars will be bet illegally across the country on the basis of the point spread established here.

What the bookmakers need, of course, is a point spread that will attract roughly the same amount of money to each team, and that is why the Dolphins have been made a solid three-point choice. Bob Martin, the city's most influential oddsman, proposed this number, and most bookmakers are using it, even if they do not agree. It reflects the reality of a game that most experts view as a tossup.

"I feel like Washington should really be the favorite," said Buddy Shannon, manager of the Santa Anita Sports Book. "But nobody believes in them. Last week, everybody was taking the Cowboys against them.

"The public is going to bet the Dolphins. They're a team you always get a lot of action on. They're sound and they're solid and they've got Don Shula and the public is going to bet them."

Sonny Reizner, oddsmaker at the Castaways, concurred. "The Redskins are an impressive club," he said, "and professionals appreciate them. But from the point of view of the general public, they don't have charisma."

Reizner thought that public support for the Dolphins could be so strong that it would force the point spread to 3 1/2 or four points. But if that happens, he said, the sharp professional gamblers would quickly leap in and take the Redskins.

Bettors here have the opportunity to leap in on just about any aspect of the Super Bowl--not just such tame matters as who will win or lose. At the Dunes Hotel, you can bet whether Joe Theismann will pass for more yardage than Don Strock and David Woodley combined. You can also lay 6 to 5 that Mark Moseley will kick more field goals than ailing Uwe von Schamann. (That proposition looks so inviting that there must be a catch).

At the Castaways, Reizner will let you bet whether there will be more or fewer than three interceptions in the game. You can even bet whether the total of field goals, interceptions and lost fumbles is higher or lower than eight.

Even with all the action that the game and these gimmicks will generate, Reizner said that bookmakers' business will suffer slightly because the Redskins rather than the Cowboys advanced to the Super Bowl. "Those Texans really like to come here and bet on them," he said. "Washington, D.C., doesn't seem like the betting capital of the country."

He should try telling that to a Washington bookie. When the Redskins beat the Cowboys last Saturday, most members of that profession in the nation's capital suffered the worst shellacking of their careers. They were caught in a crossfire. Rah-rah fans bet the Redskins, of course. Objective handicappers, who realized that the Cowboys did not deserve to be favored at RFK Stadium, bet on the home team, too.

"I've never seen anything like it," said one small-scale bookie. "Once in a while I might be overloaded by $3,000 or more on one team. In that game, I had $14,000 on Washington and $250 on Dallas." Larger-scale bookies took a proportionately larger beating.

So members of the Washington bookmaking community were probably quaking when the official Vegas line made the Redskins, getting three points, look like such an attractive betting proposition. While they will certainly adjust the line--"I think Miami by one point would be a good number there," Reizner said--there is little they can do to stem the tide.

In Las Vegas, bookmakers can make slight adjustments to the point spread to ensure themselves of a profit regardless of the outcome. In Washington, bookies can only brace themselves for the deluge.