Consider Joan Quigley`s new book, ''What Does Joan Say?'', as Part 3 of the Star Wars Trilogy. The other Star Wars Trilogy. The one that began in May 1988, when former chief of staff Donald Regan`s ''For the Record'' revealed that Nancy Reagan said yes to astrology. And in a big way. A San Francisco astrologer had called the shots on ''virtually every major move and decision the Reagans made.''

Part 2, Nancy Reagan`s ''My Turn,'' published last year, pooh-poohed Regan`s account. Basically Nancy said, Hey, my husband was shot, I got nervous, so I turned to an astrologer. All she did was pick a couple of dates for a couple of speeches. What`s everybody making such a big fuss about?

''Joan`s recommendations had nothing to do with policy or politics-ever,'' wrote Nancy Reagan. ''Her advice was confined to timing.''

Not true, says the astrologer in Part 3: ''What Does Joan Say?'' (Carol Publishing group, $17.95). Timing was just the beginning.

''What she left out about astrology would fill a book,'' says Quigley.

''That`s why I`ve written this one. Through Nancy I had a direct line to the president.'' And Quigley used it, she says. For seven years, she boasts, her astrological advice controlled everything at the White House from the date of the president`s cancer surgery to his attitude toward the Soviet Union.

Many of her claims are backed up by Donald Regan.

''The President`s schedule and therefore his life and the most important business of the American nation was largely under control of the First Lady`s astrologer,'' Regan has written. Although he first thought White House aide Michael Deaver, a confidant of the Reagans, was kidding when he told him about the astrologer, he later learned that ''the long-established floating seance'' was nothing to laugh at. Regan kept a color-coded calendar on his desk marked with Quigley`s ''occult prognostications:'' green for good days, red for bad days, yellow for iffy days. They controlled the times the president could

''speak, move or commence negotiations with foreign powers.''

If Nancy Reagan was the power behind Ronald Reagan, it appears Joan Quigley was the power behind Nancy Reagan.

No time for her own stars

Joan Quigley doesn`t wear a turban. No big dangling earrings. She doesn`t have a crystal ball. She`s actually quite an elegant lady, with silvery blond hair, a conservative wardrobe, an aristocratic manner, an independent income and good jewelry. She`s a San Francisco socialite and a staunch Republican who just happens to read horoscopes. She has been reading Nancy Reagan`s since 1973. During the last three months of the 1980 campaign, they communicated constantly. But they lost touch until May 1981.

That`s when Nancy Reagan called with two questions: (1) If Quigley had been following the president`s horoscope, could she have warned him about the assassination attempt? (2) Could she help Nancy improve her image? She answered yes to both, and that renewed a relationship that lasted until 1988. For seven years, Joan Quigley says, she was consumed with Ronald and Nancy Reagan`s horoscopes. She was in touch with Nancy sometimes two or three times a day. Their phone calls would frequently last for hours. She put her career and personal life ''virtually on hold during that period.''

Quigley was too busy even to look at her own horoscope. ''I was in the library. It has a parquet floor and I slipped on it and fell,'' she says.

''The next day, I fell again. I thought: `I never fall. What`s wrong with me?` So I looked at my horoscope and there was Mars opposing my sun, which causes accidents. But I had just been too busy to even look.''

Now, you may or may not believe in astrology. You may or may not think Mars can make someone trip. It doesn`t really matter. What matters is that Nancy Reagan believes it. And it appears Ronald Reagan does too.

Through Nancy, he apparently sought Quigley`s advice on presidential matters. Quigley says he asked for her opinion on his meeting with Mikhail Gorbachev at Reykjavik and when Irangate would be over.

Joan had plenty to say, and the Reagans listened and so, of necessary, did three chiefs of staff, James Baker, Regan and Howard Baker. Vice President Bush learned about it shortly before Regan resigned. Regan says that ''Bush uttered what was for him a strong expletive: `Good God, I had no idea.` ''

Quigley was allowed to overrule the president`s doctors and delay his colon cancer operation for three days. She was allowed to overrule Lee Atwater, then Reagan`s deputy campaign manager, and postpone the president`s announcement of his candidacy for a second term for nearly seven weeks. She was allowed to overrule Donald Regan and keep the president away from the public and the media during the Iran-contra scandal for months.

She was allowed to pick the times for takeoffs and landings of Air Force One, the dates for news conferences, speeches, State of the Union addresses, debates, trips and announcements. She did horoscopes for people the president dealt with, including Cabinet members. She spoke directly with Deaver and

''made a policy decision'' concerning the president`s trip to the cemetery at Bitburg where Nazi soldiers were buried. She, also, ''contributed ideas and astrological advise that shaped administration policy with the U.S.S.R.''

Before the president`s first meeting with Gorbachev in Geneva, he was in his ''evil empire'' period concerning the Soviet Union. But Gorbachev`s horoscope convinced Quigley that he shared ''an Aquarian vision of the brotherhood of man'' with Reagan. She saw such ''formidable chemistry''

between the two men`s charts, that had they been man and woman, they would have fallen head over heels in love. Quigley believes they knew each other in another life.

So she says she talked the president, through Nancy, into going to Geneva with an open mind.

''Do me a favor,'' she told Nancy. ''Get Ronnie to sell arms reduction, democracy, human rights and free enterprise to the secretary general. If he goes to Geneva with the right attitude, he can accomplish miracles.''

`I stood guard, astrologically`