LEAD: JOHN UPDIKE performed his own bit of deviltry when he concocted ''The Witches of Eastwick,'' a novel filled with such delicious mischief that it seems made for the movies. With its three small-town New England witches, all divorced or conveniently widowed (one has ''permanized'' her husband ''in plastic and used him as a place mat'') and its latter-day devil, an extremely eligible bachelor who adores synthetics and Pop Art, Mr.

JOHN UPDIKE performed his own bit of deviltry when he concocted ''The Witches of Eastwick,'' a novel filled with such delicious mischief that it seems made for the movies. With its three small-town New England witches, all divorced or conveniently widowed (one has ''permanized'' her husband ''in plastic and used him as a place mat'') and its latter-day devil, an extremely eligible bachelor who adores synthetics and Pop Art, Mr. Updike's story has great comic possibilities. It also has enormous visual appeal, since the witches are apt to do things like unstring pearl necklaces or conjure up thunderstorms to express their pique.

Furthermore, it's sexy, being the story of how the playfully diabolical Darryl Van Horne lures all three witches into his hot tub for extended sybaritic celebrations. But from the film maker's standpoint, ''The Witches of Eastwick'' is also something of a trap. Its best moments are decidedly uncinematic, recording the innermost thoughts of the various characters or the remarkable auras they perceive around one another, not to mention those extended hot-tub scenes. More importantly, Mr. Updike's own genius for astonishingly fine-tuned description dwarfs anything a camera can do.

So in making a film of ''The Witches of Eastwick,'' George Miller (director of the ''Mad Max'' films) and the playwright Michael Cristofer (who wrote ''The Shadow Box'') have had to start from scratch. They have radically altered things, and their film resembles Mr. Updike's novel only remotely; even the characters' names have been rearranged and spelled differently. In the face of such sweeping changes, comparisons with Mr. Updike's book would be invidious, unfair and unnecessary if the film's own sense of purpose were clear. But it isn't; in fact, it's not even clear for whom the film is intended.

''The Witches of Eastwick,'' which opens today at the Beekman and other theaters, brings a broad, obvious, punchy style and a lot of special effects to bear upon a story of seduction that is much too frail to support this kind of gimmickry. The chief thing that has been lost, for all the levitating and vomiting and so on, is the sense of witchiness itself. Mr. Updike's witches knew their own powers, and their shrewdness was a large part of the fun. But in the film, it's never clear what Alexandra (Cher), Sukie (Michelle Pfeiffer) and Jane (Susan Sarandon) know about the supernatural, or even about one another. These three characters are conceived here only in the sketchiest way. All we know is that they are a hairdresser's delight, with their respective cascades of black, blond and reddish curls, and that they have collectively made a wish for Mr. Right.

This brings them Daryl Van Horne, in the form of Jack Nicholson, whose high spirits and madman antics are always a treat, even when they've become more or less familiar. The film's flamboyant, mock-elegant costumes for Mr. Nicholson are more or less novel, as are some of the settings; it can't help but be fun watching him float in an inflatable pool zebra while spitting cherry pits into a huge silver bowl. A lot of what he does here recalls ''The Shining,'' but the reprise is enjoyable.

Mr. Nicholson has the chance to deliver three wildly excessive amorous pitches, one tailor-made for each witch, and an inspired tirade asking why God created the opposite sex (''So what do you think? Women - a mistake? Or did He do it to us on purpose?!''). Typical of the film's excesses is the fact that this tantrum, so exuberantly delivered, is followed by an ugly, unnecessary fit of vomiting in a church.

Like a lot of the magical events that occur here, the vomiting is none too well explained. In the book, Mr. Updike's witches decide to torment an obnoxious neighbor (with whose husband one of them is having an affair) by putting feathers, straw and various household sweepings into a container and forcing this bewildered woman to spit them out.

In place of this, the film offers Veronica Cartwright as a shrill hysteric who spits pits as the witches eat cherries. But the performance is overwrought, the sexual motive is missing and the effect is as puzzling as it is unpleasant.

Even when the witches and Van Horne embark upon a tennis game in which magic clearly governs the ball, it's not entirely clear who is doing what to whom. And a lot of the tricks have been appropriated at random and simply reassigned to different characters or different situations.

''The Witches of Eastwick'' does have enough flamboyance to hold the attention, directed as it has been by Mr. Miller in a bright, flashy, exclamatory style. But beneath the surface charm there is too much confusion, and the charm itself is gone long before the film is over. Though the performers are eminently watchable, the sight of all three women flouncing naughtily in their lingerie, or marching about in unison, effectively renders the whole thing rather silly. In any case, none of them seem a match for Mr. Nicholson's self-proclaimed ''horny little devil.'' As battles of the sexes go, this is barely a scrimmage. Giving Devil His Due THE WITCHES OF EASTWICK, directed by George Miller; screenplay by Michael Cristofer, based on the book by John Updike; director of photography, Vilmos Zsigmond; edited by Richard Francis-Bruce; music by John Williams; production designer, Polly Platt; produced by Neil Canton, Peter Guber and Jon Peters; released by Warner Bros. At Criterion Center, Broadway and 45th Street; 84th Street Six, at Broadway; Beekman, Second Avenue at 65th Street; Gramercy, 23d Street near Lexington Avenue; Movieland Eighth Street, at University Place; Movie Center 5, 125th Street, between Powell and Douglass Boulevards. Running time: 122 minutes. This film is rated R.