In the wake of their defensive decline this season, the Redskins will:

* Concentrate their high draft choices on strengthening the defensive line and cornerback positions.

* Consider seriously the possibility of switching to a 3-4 defense next season, at least during some game situations.

Coach Joe Gibbs admitted that the Redskins will look closely at the 3-4 (three linemen, four linebackers) in the offseason, although he says his "gut feeling" now is that the team will stay with its present 4-3 alignment in 1982.

"Looking at our personnel on defense, we have the most depth at linebacker, and that makes you think about a 3-4," he said. "There is no question we will look at it seriously but it would take a lot to make us decide to use it."

The Redskins' draft priorities will be determined, according to General Manager Bobby Beathard, "after we have Joe's evaluation of what he thinks we need. It wouldn't surprise me if we'd go in looking for defensive help, like we did last year. There aren't an overabundance of linemen this year; we'd just have to find some people others have overlooked. You are always looking for wide receivers and cornerbacks, too."

Gibbs also declined to be more specific. But it has been learned that the Redskins have made a preliminary decision to use their first pick in the draft, which comes on the second round (they traded their No. 1 choice to Los Angeles last April), for a defensive lineman.

They would also like to pick up one or two cornerbacks, since both Joe Lavender and Lemar Parrish are in their 30s, plus another outstanding wide receiver to accompany Art Monk. And they are looking for more tight ends to be used in Gibbs' two-tight-end offense.

Even with the absense of a first-round pick, the Redskins would like this draft to accomplish for the defensive line what last year's did for the offensive line

Last April, General Manager Bobby Beathard selected tackle Mark May, a starter for half the season, in the first round; guard Russ Grimm, a starter the entire year, in the third, and guard Darryl Grant, a potential starter, in the ninth. Beathard later signed free agent Joe Jacoby, now a starting tackle.

Washington has nine draft selections in 12 rounds, although Beathard says the team will actively pursue offseason trade possibilities that may increase their total number of choices.

The Redskins never expected their defense to be among the top four or five in the league this season. They knew they lacked the overall talent and the pass rush to be that good. But both Gibbs and Petitbon thought the unit would be better than it has been.

After 13 games, Washington is ranked in the middle of the league in terms of yards surrendered (329 a game), but is a dreadful 24th in points surrendered (315). At its current pace, the defense could become the Redskins' worst since 1968. The unit has already given up more points than any Washingon squad in 11 years.

The reasons for the defense's decline are three: injuries, lack of overall quickness and unexpected personnel developments.

Injuries to Parrish and to linebacker Monte Coleman, perhaps the defense's two best athletes, have reduced substantially their contributions this season. Parrish, an outstanding pass defender and eight-time Pro Bowler, has only one interception and has not played the last three weeks because of a sore knee. Coleman sat out five games with a fractured shoulder and has struggled in his role as a big-play producer. Linebackers Brad Dusek and Rich Milot also have missed at least four games with injuries.

The front four's lone pass-rushing threat, Coy Bacon, was dismissed before the fourth game because of disciplinary problems. Fred Cook and Wilbur Young, two offseason trade acquisitions, failed miserably as pass rushers and were cut.

The lack of a pass rush exposed a surprisingly inconsistent secondary to the long pass. The line and linebackers also haven't been able to cope with sweeps, especially to the right side of the Redskin defense, where rookie Dexter Manley now is the regular at end.

Manley and Mike Clark, now on injured reserve, both have quickness and figure in the team's future, as does rookie end Mat Mendenhall. But the team has no depth at tackle, where Dave Butz will be 32 next season and Perry Brooks is inconsistent.

Who the Redskins obtain in the draft also could influence their thinking about the 3-4, the alignment used by 17 of the league's 28 teams this year, including 10 of the top 14 defensive clubs.

Both Gibbs and defensive coordinator Richie Petitbon prefer the 4-3. They are aware that Butz, who would become a nose guard in a 3-4, hates that position. But they also know that Coleman, Milot, Neal Olkewicz, Mel Kaufman, Quentin Lowry, Dusek, Charlie Weaver, Pete Cronan and Larry Kubin provide the depth at linebacker needed for a 3-4, especially if the line can't mount a pass rush.

"You do what your personnel dictates," Gibbs said. "I went to a one-back offense this year, so I'm open to changes if I think it's the way to go."