THREE teen-agers from Detroit -- Diana Ross, Florence Ballard and Mary Wilson--were molded by Motown mastermind Berry Gordy into a pop institution, the No. 1 female vocal group in the world with an unbroken chain of international hits.

But when Diana Ross left things were never the same. Although the Ross-less Supremes had '70s hits, including "Stoned Love" and "Up the Ladder to the Roof," the group gradually faded from the charts.

Still, the Supremes have kept going for 30 years. Wilson, the only original member still in the act, kept the group intact and on the road, and she has coached three lineups of new Supremes. Cindy Birdsong replaced the late Florence Ballard and Jean Terrell replaced Diana Ross, Sherrie Payne (singer Freda Payne's sister) and Susaye Greene (of Wonderlove) were another incarnation. Now the Supremes are Wilson, Robin Alexander and Karen Jackson.

Taking the lead after working with pop icon Diana Ross was quite an adjustment, Wilson says. "First of all, I used to sing all the background ooohs and aaahs and then all of a sudden I was singing lead. But remember, we all sang lead when the group was formed, so singing lead was not really new to me. It was a major adjustment, but it's like driving a car, it's just a different gear."

Wilson says she and Ross are "still very close. We still have our spats, but I know her very well, and she knows me." Birdsong is now singing gospel music in Los Angeles.

"I kept a diary through all those years with the Supremes," says Wilson, who has been working her diaries into a book about growing up as a Supreme. Several books about the Motown hit machine have been written by outsiders, but no Motown artist has told his or her story yet. "It's partly because of 'Dreamgirls,' " she says. "People keep coming up to us and asking if that's the way it was."

"Dreamgirls," Michael Bennett's hit Broadway musical, bears more than a passing resemblance to the story behind the Supremes' rise to the top of the pop and soul charts. Ross has so far refused to see the show, which recently opened in Los Angeles, but Wilson says she loved it and has seen it on both coasts. "I can't say it was accurate," Wilson says. "But I loved the idea. They used us as a pattern to talk about what happened with music in the '60s." Ford's Theater opens its new Motown-inspired revue, "Dancin' in the Streets," Tuesday.

The current group still tours often--it just returned from a three-week visit to Asia and will appear at the Sands Hotel in Atlantic City tonight and tomorrow. They will appear at the Capital Centre June 4 with Frankie Valli's Oldies Show. "I'd be silly to sit down--I'd die. And, of course, you get accustomed to a way of life. I could never make this kind of money doing anything else." The 1983 Supremes still deliver medlies of the old hits with lots of Vegas glitter and costume changes.

Last year, to mark her 24th anniversary as a performer, Wilson took her first break from touring for a vacation with her husband and manager, Pedro Ferrer. During that hiatus she studied acting, auditioned for some Broadway shows and made some television commercials. Now Wilson, who like many of the Motown artists says she was neglected by the label, is hunting for a solo recording contract.

Motown recently celebrated its 25th anniversary with a gala performance, taped for television, and three of the first Supremes--Ross, Birdsong and Wilson--appeared to sing a brief "Someday We'll Be Together." Ross, apparently miffed because Wilson was upstaging her, jostled Wilson on stage, then Smokey Robinson jumped onstage to sing along and smooth things out. "It wasn't a hassle between us," Wilson says. "I must have just gotten in the way and she pushed me a little. It wasn't intentional." Wilson says television viewers probably won't see that incident on the television special.