When the superstars of American fashion speak . . .

People sit up and pay attention to what Ralph Lauren, Perry Ellis, and Calvin Klein are designing. These superstars have become the stellar influences in American fashion. Their clothes, aimed at the young-minded upscale customer, reflect an intuitive sense of what that customer will want.

Next spring and summer, they think she'll want sportswear again - but not, of course, the same as before. All three designers show romanticized but simplified versions of favorite classics in their 1984 collections. Although their approaches have differed in other seasons, this time they agree on easy, clean-cut dressing as the ideal for today's sophisticate.

White linen, often paired with natural, takes the honors. Nothing looks fussy. Necklines are uncluttered. Skirts are a graceful length - sometimes to the ankle. Heels are low. Accessories are minimal. Klein's models, who in previous years have been earringed and braceleted to the nines, wore no jewelery - just a hat with a broad curled-up brim.

The mood is pretty much the same, too. All three designers have chosen far-away and long-ago places as atmospheric points of reference. Both Lauren and Ellis have gone back to the bush, but on different sides of the globe. Lauren's locale is colonialist Africa; Ellis's the Australian outback. Some of Klein's new fashions reminded observers of an imaginary early-days Wyoming or a mythical Southwest.

In spite of the undercurrent of nostalgia, the clothes are very up to the minute. The music at Lauren's opening was the sound track of the PBS series ''The Flame Trees of Thika,'' a story of a Victorian family on a plantation in Kenya. But the clothes that appeared in his showroom will look right next year on America's city streets or country lanes.

His short-sleeved safari jackets of raw silk with white crepe-de-Chine pleated skirts, for instance, will fit neatly into a modern wardrobe. So would the new cable-knit cardigans, made of yarn that has a flaxen look, and the Norfolk jackets of nubby linen or silk, worn with madras or rose-printed skirts.

The color scheme at Lauren centers around white and goes from there to beige, British tan, dusty pink, and coppery rust. This designer likes antique touches, which for spring consist of handkerchief-linen camisoles or blouses delicately tucked and inset with lace. Some of them were shown under old-fashioned waistcoats that had gold watch chains looped across the front. But the new accessory is a woven leather belt. It's fairly wide and has a silver buckle.

New pants, mostly of linen, are shaped like jodhpurs. The tissue-weight suedes include loose tops, bush skirts, and a safari dress in taupe. Biggest hits at Lauren are the white-linen trench coat and the natural-linen duster - each of them generously proportioned, though impractical, perhaps, when you think of the cleaning bills.

Perry Ellis has wonderful oversized linen coats, too. He calls them ''Flinders range coats,'' in keeping with his Australian theme. Again of linen, but in shell pink and other watercolor pastels as well as white and natural, they cover pale silk crochet sweaters with picot edgings.

Ellis still likes the look of short jackets with fly-away backs over high-waisted skinny skirts. But his selection of styles is varied. Bushwhack suits composed of striped or herringbone linen crash blazers with walking shorts , as well as checked riding pants with silk sweater-jackets are among several options.

With an eye on the next America's Cup races, Ellis also has a series in navy and white. Punctuated with pinstripes or coin dots, this group comes in crepe de Chine besides the omnipresent linen. Intarsia handknits are dotted, striped, or diamond-patterned for both men and women. Ellis presented menswear, too, at his showing. Some of the women in the audience said they coveted the man's dotted silk sport shirt.

Everything at Calvin Klein is long and lean with no nonessentials to get in the way. But the sportswear syndrome is present, particularly in the poplin trench-coat dresses, which were much admired by buyers.

Fashion designers used to be dictators whose words were gospel. Today we have superstars who are out there, using their engaging looks and likable personalities, energetically marketing styles of living. The hype may sometimes be intrusive (confronted with billboards advertising his new underwear at every bus stop, New Yorkers can hardly ignore Calvin Klein).

But when the fashion concepts are as timely as those of Lauren, Ellis, and Klein, they motivate others in the business to follow their leads. It may take six months or a year, but their brands of chic will eventually filter down. Before you know it, we may all be in poplin trench dresses or linen bush jackets.