The first thing to understand about this third preseason game of the year, between the Miami Dolphins and the Washington Redskins Friday night at 8 in soldout RFK Stadium, is this:

"The only time you have Super Bowl rematches," said Miami Coach Don Shula, "is when you have Super Bowl rematches."

Said Redskins Coach Joe Gibbs: "It will create some excitement and sell more tickets with the two Super Bowl teams playing, but it is a preseason game."

The game will be nationally televised by ABC, but will not be shown locally because the sellout wasn't reached 72 hours in advance.

After a light practice in a light rain this morning, the Redskins broke training camp, anxious to return to Washington after five weeks of two-a-day practices and challenges, both off and on the field, to their Super Bowl-winning stability.

"Overall, I'd say it was a good camp. Of course, some things that happened, you are disappointed with," said Gibbs. "But at least we didn't have any large, major injuries."

For the Redskins (1-1), opportunity rests Friday night with rookie quarterback Babe Laufenberg, first-year strong safety Ken Coffey and second-year left defensive end Todd Liebenstein. Liebenstein was told Monday that he would start against Miami. His playing time will likely be more than be expected because Mat Mendenhall, starting left end last year, left camp Wednesday and flew home to Utah, without notifying team officials.

Gibbs said today that Laufenberg, drafted in the sixth round from Indiana, will replace starter Joe Theismann. Gibbs did not say how long his starters will play Friday.

"I think we're looking at Babe's overall performance, the confidence he exudes. We know he's got a great arm," Gibbs said.

Laufenberg spoke of "the mental part of my game" on which he must work before getting down to more important matters: "If Howard Cosell mispronounces my last name," he said, with a rookie's blush, "my mother will give him a call."

Richie Petitbon, coach of the Redskins defense, has more crucial worries. "Our No. 1 concern, of course, is Coffey," he said. Coffey is replacing all-pro Tony Peters, arraigned Wednesday on a cocaine trafficking charge.

"He needs the most work. This game is going to determine whether or not we think he can play. It's a good test for him . . . He's a good athlete. His weakness is inexperience. He'll be learning at our expense, which we don't like to do," Petitbon said.

Coffey, a shy, confident 23-year old Texan, who has intercepted two passes in the preseason, said, "Maybe I'm being compared to Tony (Peters)--or maybe I'm just being pushed to be the best. In this case, the two are synonymous, because Tony was the best there was . . . I think I am doing okay, but I think I can do better."

Darrell Green, top draft pick from Texas A&I, will make his first start at left cornerback, position of holdout Jeris White. "It's the start of my move toward my goal--starting," he said.

Green may confront new competition next week. The Redskins acquired Anthony Washington, a third-year cornerback, from the Pittsburgh Steelers Wednesday night in exchange for an undisclosed 1984 draft choice.

Washington, who will not play Friday, started four games during his rookie season with the Steelers.

The Dolphins (0-2) have not had a successful preseason. They lost their opener at Dallas, 20-17, when the Cowboys scored with two seconds remaining. Then last week, the Dolphins lost a 14-0 lead, before losing to New Orleans, 19-17, on a field goal with four seconds left.

The Dolphins are quick to note that in neither frantic finish was their first-team defense on the field. So far, it's been the kind of fickle preseason for Miami where some of the loudest cheers in the Orange Bowl have been addressed to a rookie punter. Reggie Roby, drafted in the sixth round from Iowa, has averaged 54 yards after six punts.

In the first two games, Shula played starting quarterback David Woodley for the first quarter, then brought in reserves Dan Marino and Jim Jensen. Woodley is expected to play slightly more against the Redskins, perhaps the entire first half.

The Redskins placed rookie offensive tackle Bob Winckler on injured reserve today to again reach the 70-man roster limit. Winckler suffered a neck injury this week.