Paramount Pictures held its chief prerelease screening of "Clue" last night, but the studio didn't show the movie in its own theater or in any of the public theaters it usually uses. Paramount needed to screen "Clue" at Mann's Westwood Complex because it has three screens -- and "Clue," as has long been rumored, has three endings.

Beginning tomorrow the public will get to choose whether the culprit's Colonel Mustard in the kitchen with a knife or Miss Scarlet in the library with a candlestick. (Those, by the way, are not necessarily two of the endings.)

To make sure moviegoers won't have to attend endless showings of "Clue" to catch all three endings, theater listings will indicate which of the three is being shown at each theater. (No, the ads won't tell you who dunit; instead, the versions will be coded A, B and C.)

And if the idea of having to see the movie three times to view it all is annoying, you can always wait until it's released on videocassette. In that configuration, all three endings will be supplied. The Box-Office Summit

It's probably more a coincidence than a case of postsummit interest, but last weekend's top three films all deal with the Soviet Union. In first place, naturally, was "Rocky IV," in which the intrepid Italian Stallion climbs into the ring opposite the best Soviet science could create. The film didn't maintain the pace set by its record-breaking first weekend; its per-screen average, for example, was cut nearly in half. Still, the movie did make a rousing $11.2 million to bring its two-week total to almost $50 million -- and that number will get lots higher by the time Christmas rolls around, and MGM/UA floods the market with prints of "Rocky IV." By then, it will be in 2,135 theaters, 1 out of every 10 in the United States . . .

The weekend's runner-up movie is "Spies Like Us," in which Chevy Chase and Dan Aykroyd venture into the U.S.S.R. to ineptly further the cause of peace; it made a healthy $8.6 million despite mostly negative reviews. And in third place, "White Nights," in which Mikhail Baryshnikov and Gregory Hines try to dance their way out of Russia. The movie isn't hurt by the fact that it's the first movie since "Saturday Night Fever" to have two of its songs -- "Separate Lives," by Phil Collins and Marilyn Martin, and "Say You, Say Me," by Lionel Richie -- in the top three of the pop music charts.

Incidentally, this makes two consecutive weekends in which the top three movies have been related. The previous weekend all three somehow dealt with Christmas: No. 3 was "One Magic Christmas," No. 2 was "Santa Claus: The Movie" and the winner was "Rocky IV," which qualifies if you count that the climactic slugfest takes place on Christmas Day. Brits Go Back to the Movies

Stateside movie makers moaning about this year's lackluster business can't even look to Great Britain with feelings of superiority anymore. British moviegoing hit an all-time low last year, but now that U.S. attendance is off, Great Britain is seeing its healthiest business in years. Estimates say 1985's business could be up by a third to a half. American filmmakers who attribute their lost sales to the home video boom, though, can take heart: In Great Britain, video business has peaked, and people are leaving their living rooms and returning to the theaters . . .

David Obst, who won a Pulitzer Prize when his Dispatch News Service helped uncover the My Lai massacre and later became one of Washington's top literary agents, has moved into less lofty movie business pursuits. Obst is one of the three executive producers and screen writers for "Whoopee Boys," a so-called "slobs versus snobs" comedy from the creators of "Revenge of the Nerds." Michael O'Keefe and Paul Rodriguez are the title characters in the film, which began shooting early this week . . . Jack Nicholson is reportedly considering the lead role in "Ironweed," the film of William Kennedy's Pulitzer-winning novel. Kennedy -- who helped on Francis Coppola's "Cotton Club" -- has written the script for Brazilian director Hector Babenco ("Kiss of the Spider Woman," "Pixote"), and Nicholson liked it enough to tell Daily Variety, "I would like playing an Irish bum."