FACE TO WATCH: HOLLY ROBINSON

So what if everyone says Hangin' With Mr. Cooper is a rip-off of Three's Company? ''If we're going to be as successful as that show,'' says the voluble Holly Robinson, ''they can make fun of us all they want.'' Making fun herself of comments that she is simply taking over where Suzanne Somers left off, she quips, ''I do plan on marketing the black version of the Thighmaster.'' Actually, she may not have time. On top of a budding musical career (she and costar Dawnn Lewis recorded Cooper's theme), she'll play Diana Ross in the fall ABC miniseries The Jacksons: An American Dream-doing her own vocals. And she'll need hours to squirm into the snug frocks she'll wear as Cooper's upwardly mobile secretary Vanessa. ''I don't have a problem being the show's T& A,'' she says, ''because black women are rarely shown this way.'' A child of the biz (her mom, Delores, is a talent manager, and her dad, Matthew, played Sesame Street's Gordon and was a Cosby Show producer), Robinson has studied at the Sorbonne and is fluent in French, Italian, and Spanish. On the romantic front, she is no longer engaged to actor Brian Robbins (Head of the Class) and is, for the record, dating again. Robinson is best known for starring as a gun-toting undercover officer on Fox's hit 21 Jump Street. But since the show revolved around Johnny Depp, some people have forgotten her. On one audition, she says, she was told, '''Sorry, but we just don't see you holding a gun.' I thought, 'What? Excuse me?''' She got the same doubtful reaction when she asked to try comedy: ''We know you can break up female gangs and fire a gun. But can you be funny?'' Robinson plans to have them eating their words-while they laugh.