NEW YORK (AP) _ ABC leaped up from its third-place rut to tie NBC in this week's television ratings, according to the A.C. Nielsen Co., and might have won the race but for the sagging popularity of the Academy Awards telecast.

''The movie audience isn't necessarily a television audience, and that's reflected in the lower level of viewing,'' George Keramidas, ABC's vice president for research, said after figures released Tuesday showed the Oscar show with it lowest rating ever.

The Oscars on ABC finished third for the week with a rating of 27.3, or 43 percent of the viewing audience. ''The Cosby Show'' on NBC again topped all the competition with a 32.0 rating and 54 share, followed by NBC's ''Family Ties'' with a 28.5.

Barbara Walters' hour-long interview with President and Mrs. Reagan before the Academy Awards finished 10th with a rating of 20.9, but she pulled a 25.2 rating before the Oscars last year, when the subjects included Boy George.

NBC had two other shows in the Top 10: ''Cheers'' was sixth with a 22.3 rating and ''Golden Girls'' was seventh with 21.8.

In addition to the Oscars and Ms. Walter's special, ABC had ''Who's the Boss'' ranked fifth at 22.5 and the premiere episode of ''Perfect Strangers'' eighth at 21.3.

CBS' ''60 Minutes'' finished in a tie with the Walters special, while ''Murder, She Wrote'' was fourth for the week at 23.7 and ''Mrs. Delafield Wants to Marry,'' a TV movie starring Katharine Hepburn, ninth at 21.0.

Each rating point represents 859,000 homes; the share reflects the portion of the set in use during a particular time period.

For the week, NBC and ABC each had a rating of 15.8, while CBS lagged at 14.2 and finished third for the first time since November. With three weeks to go in the 30-week prime-time season, NBC leads with an average rating of 17.7, followed by CBS with 16.7 and ABC with 15.0.

As recently as 1983, the Oscar ceremonies claimed a 38.0 rating and a 59 share, still short of the glory days. in 1974, the Oscars got 78 perent of the audience and in 1960 they audience share was 81 percent, according to NBC.

''Something like the Academy Awards very often is a function of the titles and the people up for the awards and their appeal,'' said Keramidas. Films which do big business, such as the ''Rocky'' epics and ''Porky's,'' don't get many nominations for Academy Award, he noted.

''It isn't helpful for the Osars to be on the network doing poorest in overall ratings,'' said Gerald Jaffe, NBC's vice-president for research. A network doing well, he said, tends to do just a bit better in ratings on any of its shows.

''I don't know how much that inertia is worth,'' Jaffe said, ''but it's there.''

''Perfect Strangers,'' a comedy of clashing cultures starring Mark Linn Baker and Bronson Pinchot, got an obvious boost from follwoing the hit series ''Who's the Boss.''

''We'll have to watch it in the weeks ahead to see how it holds up,'' Keramidas said.

Although it's unusual for a first episode to crack the top 10, it isn't unprecedented, nor is it a guarantee of success. In 1979, ABC spun off ''The Ropers'' from ''Three's Company,'' and the new show started in second place in the Nielsen list. By the end of the season, it was near the bottom and was canceled.

''Perfect Strangers'' got one-third of the audience in its time slot, while its competition on CBS, the debut of ''Morningstar/Eveningstar,'' attracted only 15 percent of the audience and a rating of 9.7

Mary Tyler Moore's new show, ''Mary,'' and ''Foley Square,'' which CBS moved to Tuesday night in search of an audience, finished 58th and 60th for the week. ''Mary'' had a rating of 9.8 and ''Foley Square'' 9.0, about half the audience which ABC's ''Moonlighting'' and NBC's ''Hunter'' each captured in the same hour.

ABC got encouraging ratings for two new shows on Friday: ''Mr. Sunshine'' finished 44th for the week with a 13.2 rating and 23 share, and ''Joe Bash'' was 49th with an 11.3 rating and 19 share. Both, however, had the luxury of opening on a night when ''Dallas'' had been pre-empted on CBS.