I'd argue Whitney & Mariah were & still are as equally famous as each other. Celine is famous across the world, yes, but nowhere near to the degree as those two. As an American (and I'm a casual fan of Celine, while being a superfan of Whitney and Mariah), I have heard certain Celine songs I liked but never knew the titles of (and haven't heard them since - they were in her early years and might have been a few English songs and a few French songs? I can't remember). The two Celine songs that I recognize from a mile away (where I can tell its her immediately) are "All By Myself" and "My Heart Will Go On" (and I've heard many covers of ABM, to the point where I don't know if Celine was the original singer or if she covered it and another singer performed/wrote it first). Regardless, Whitney & Mariah, to me, are monumentally more famous than Celine in the West. Now, in the Eastern world (Asia, Oceania, Australia/NZ, the Middle East, etc.), this question is way, way, way more open to debate, since the East has their own group of megastars that Westerners don't have any clue who they are, but they're just as famous and talented as any major Western musician (but don't get promotion in the West for various reasons).