Already, at the age of only 3, Athena Onassis is a leading contender for the title of "richest woman in the world" early in the next century.

The sudden death of her mother, Christina Onassis, has left Athena the heiress to a fortune estimated at $1 billion.Athena Onassis thus qualifies as a "poor little rich girl," the tag once bestowed by the tabloid press on other young heiresses to great wealth. Starting in the early 1930s there were three in particular who captured the public's fancy: Doris Duke, Barbara Hutton and Gloria Vanderbilt.

Duke, the daughter of American Tobacco Company President James Biddle Duke, was only 12 when her father died in 1925 and left her a share of his $100 million fortune. Now a semi-recluse on her large New Jersey estate, Duke made news recently by posting $5 million bail for Imelda Marcos after Marcos was arraigned on federal racketeering charges. According to the most recent Forbes magazine survey of America's 400 wealthiest individuals, Duke's fortune currently is worth in the neighborhood of $800 million.

Barbara Hutton, Duke's contemporary, inherited some $20 million at age 5. Her maternal grandfather, the source of the money, was Frank W. Woolworth, the 5-and-10-cent store magnate.

If ever there was living proof of the adage that money can't buy happiness, it was Hutton. She herself once said so in almost precisely those words. "My money has never brought me happiness," she said in 1945 after the breakup of her marriage to actor Cary Grant. "In fact, after three unsuccessful marriages, I guess I'm considered a very poor risk. You can't buy love with money."

At the same time, Hutton bristled at the "poor little rich girl" label. "I am most certainly not poor," she said. "Nothing infuriates me more than rich people who keep saying they're unhappy because they have wealth. I always tell them they should go down on their knees and thank God they have money."

Duke and Hutton, incidentally, had more in common than large inheritances at an early stage in life. Each was married briefly to Porfirio Rubirosa, a Dominican playboy who was romantically linked with many wealthy and beautiful women of his time.

Gloria Vanderbilt, the third of the heiresses who fascinated Depression-era Americans, may well have earned a fortune on her own even without the $4 million trust fund left to her as an infant. Though emotionally bruised by a fierce custody fight between her mother and a paternal aunt, Vanderbilt carved out a career for herself as an actress, painter, poet, clothes designer and, most recently, memoirist.

The Vanderbilt story is familiar to millions because of Barbara Goldsmith's best-selling 1980 biography, Little Gloria . . . Happy at Last, and the TV miniseries based on it. But even without those, Gloria Vanderbilt doubtless would have remained a celebrity if only because of her surname.

Meanwhile, still another young heiress is waiting to claim her inheritance. Elvis Presley's daughter, Lisa Marie, is scheduled to take over her father's estate five years from now, when she turns 25.