"It was the best of times, it was the worst of times . . ." reads Captain Kirk near the beginning of "Star Trek II: The Wrath of Khan," and these are unquestionably the best of times for the crew of the Starship Enterprise. In its first three days in the theaters, "Star Trek II" broke the box office records set last year by "Superman II" and challenged recently by "Rocky III," which fell just short of the "Superman" total. "Star Trek" grossed well over $14.3 million in three days at more than 1,600 theaters, beating out the man of steel by some $250,000 (though the new movie is playing in more than 200 more theaters than "Superman" did, and it has a per-screen average of slightly fewer than the earlier film). "There's not a soft spot in the country," says one delighted Paramount Pictures executive, adding that the grosses are particularly heartening because, while "Superman" played in theaters concentrated in large cities, "Star Trek" has landed "in shopping centers in the boondocks all across the country." Incidentally, the 68 theaters screening the film on extra-large 70 mm screens have captured a remarkable $33,000 per-screen average, while the averages in New York, Los Angeles and Chicago are actually larger than the figures for "Superman," which played in nearly twice as many theaters in those cities. A day-by-day tally for "Star Trek": $4.4 million on Friday, almost $5.5 million Saturday (a one-day record) and $4.4 million more on Sunday. In every case, those figures top "Superman"--which, by the way, established its record by beating the previous champ "Star Trek: The Motion Picture." Revenge is sweet.