Sex sells. Or that's what the unsigned R&B/hip-hop trio UC3 is hoping by partnering with Hooters of America. As if the eat-while-you-ogle franchise didn't have enough on its plate with 353 locations, an airline, and a golf tour, it has teamed with a Florida entrepreneur to promote the upstart girl band and their debut CD, ''I'm That Girl.'' One snag: Two of UC3 are only 17. ''We find the relationship to be appropriate,'' says manager Rob Faw, ''and not all weird or indecent.'' Okay.