Saved for the last act of tonight's "Barbara Walters Special" on ABC is Nancy Reagan, who gave the ineffable babbling Babsy first crack at a TV interview since the attempt on President Reagan's life in March. Mrs. Reagan, fighting back tears and at one point putting her hand dramatically to her heart, recalls the events and her emotions of that day on the special, at 10 tonight on Channel 7.

The theme of the special, Walters says is "Woman of the Year," but it should have been called Three Tough Dames [four, counting Baba]; in addition to Mrs. Reagan, Katharine Hepburn and Lauren Bacall get fluffed and dried by Walters, who apparently has undergone some fluffing and drying herself. In her introduction, and in two of the three interviews, her hair is newly frosted and billowy. She looks like Shelly Winters auditioning for "Charlie's Angels."

With Mrs. Reagan, Walters in whispery and solicitous, holding out her hand that the first lady might touch it when the pain of the recollections gets too great. These Walters hours may be the most soap operatic interviews since Warren Hull and "Strike It Rich" -- so damply campy it's appropriate that the program is cosponsored, an announcer says, by "the makers of Agree, Enhance, Shout, and Raid." And yet this one, like most of them, ranges from mortifying to thoroughly captivating.

After a brief camera tour of the White House living quarters as done over by Mrs. Reagan -- TV's first look -- the two gals sit on a couch for their heart-to-heart. Mrs. Reagan remembers the night she got out of the bathtub and heard John Chancellor say her husband had been elected president and Walters is tacky enough to ask, "You couldn't have been listening to ABC?" Mrs. Reagan finds that just too funny -- literally too funny, since she laughs on and on.

But then comes the serious talk about the assassination attempt. "It was the last thing in the world that was in my mind," says Mrs. Reagan of the possibility that anyone would try to end her husband's life. She recalls being told at first that her husband had not been shot -- just as the nation was told by TV and radio commentators -- and then seeing blood on his lips when finally let into his hospital room.

She pays tribute to the Secret Service agents without whom "I wouldn't have my husband" and remembers how son Ron explained to the president that breathing through a tube was like scuba diving, in order to calm him."Ronnie doesn't remember it. Too bad," Mrs. Reagan says. Now on sleepless nights she eats bananas to keep from waking the president, and as for the accused attacker, "I feel terribly sorry for his parents."

Yet she remains opposed, like President Reagan, to gun control, preferring the threat of harsher punishments as a teterrent. "I agree with the death penalty. I think that people would be alive today if there were a death penalty," she says.

Bacall, in the second interview of the show, survives the tantrum of overexposure that has engulfed her in recent years. Of her former friend Frank Sinatra, she sputters, "He has about as much humor as this floor . . . No humor, none about himself . . . I wish he'd just shut up and sing." Walters measures Bacall -- as she seems to measure all the women on this show -- by the male company they keep or have kept, pelting Bacall with questions about Humphrey Bogart and Hepburn with questions about Spencer Tracy.

Hepburn is, as always, eminently and saltily quotable. "I think I was born at a time that was enormously suitable for my type of personality." she says. "Everyone's afraid -- unless they're dumb," she declares. Among her opinions: "I never found men that thrilling," "People have become such slobs," and "I think everybody's gone mad."

Although Walters keeps trying to interrupt or talk over her, an unwise course for an interviewer [especially one whose conversations will be edited down later for maximum punch], Hepburn emerges unabashedly her cantankerous self. "I've just about done what I damn well wanted to, and I made enough money to support myself, and I ain't afraid of being alone," she growls. Who says there aren't heroes anymore?