Nancy Reagan, who pushed her husband's role of peacemaker, will share the limelight with President Reagan at the Moscow summit while she tours historic sites and meets with the wife of the Soviet leader.

As the first lady visits her hostess, Raisa Gorbachev, at their third summit meeting, people will be watching to see how they get along next week.The two women got acquainted in 1985 at the Geneva summit, having tea and holding ideological talks that showed them very much apart politically. They met again last December for a round of social activities at the Washington summit.

Now Mrs. Gorbachev will play host when the Reagans arrive Sunday in Moscow.

President and Mrs. Reagan arranged to leave Wednesday for Helsinki, Finland, where they will spend several days resting during a low-key stay before proceeding to Moscow.

Mrs. Reagan is taking along an entourage of eight staffers and helpers, including her hairdresser, "Mr. Julius" Bengtsson of Beverly Hills, who always travels with her on Air Force One, and her personal maid, Anita Castelo.

Elaine Crispen, the first lady's press secretary, said Mrs. Reagan bought some new clothes for the trip, but she will also rely on her older wardrobe during the trip. She is taking a raincoat and low-heeled shoes for lots of walking.

The first lady planned to accompany her husband to Finlandia Hall Friday when he delivers a major pre-summit address, but otherwise, her schedule is billed as private.

Soon after the arrival in Moscow Sunday, when Reagan and Soviet leader Mikhail Gorbachev go into the first round of talks, Mrs. Gorbachev will take Mrs. Reagan on a tour of the Kremlin grounds.

The women will visit the Tretyakov Gallery on Wednesday. Otherwise, they will be together on social occasions, including the banquet Monday night at the Kremlin in honor of the Reagans. Tuesday, Mrs. Reagan has planned a dinner at the U.S. Embassy residence, Spaso House, in honor of the Gorbachevs. Musician Dave Bruebeck and his jazz quartet are slated to entertain at the dinner.

Tuesday will be a special day for Mrs. Reagan when she travels to Leningrad, the former Czarist St. Petersburg, where she will get her request to see the famed Hermitage Museum, the former winter palace.

Mrs. Reagan's escort for the daylong trip to Leningrad will be either Mrs. Andrei Gromyko, wife of the Soviet president, or Mrs. Anatoli Dobrynin, who spent many years in Washington as the wife of the Soviet ambassdor to Washington.

In Leningrad, Mrs. Reagan also will take a hydrofoil ride to Petrodvorets, or Peter's Palace, touring the magnificent edifice of the pre-communist era before returning to Moscow.

After the president holds a news conference Wednesday afternoon, the Reagans will be guests of the Gorbachevs at the Bolshoi Ballet and dine afterwards at the home, or "dacha," of their hosts.