Jams.
+15 Attack
I was about to make this argument, but then I thought more her matchup spread and it's honestly not bad. The biggest issue is her awful ZSS matchup. I don't think any other top tier has a matchup as bad as Rosalina vs ZSS other than Ness, who as a result gets shunted to the lower end of top tier. That said though, her other disadvantages versus Pikachu and Falcon are only slightly disadvantageous, and perfectly winnable. Her Sheik matchup is honestly pretty good and shouldn't be held against her. She's probably one of Sheik's 10 hardest matchups, and Rosa wins this matchup decently often at high level. Sheik is just a good character that basically beats the whole cast.In my opinion, a tier list should be a visual representation of who has the best matchup spread, scaled with the meta -- getting scraped by sheik is much more meaningful than being 9:1 vs Pacman.
Using these parameters. Rosa is not top 3. She loses hard to ZSS, Sheik, Pikachu, Falcon, and perhaps others. Yes, she is very good. She is a noob killer with amazing options, frame traps, etc. She's great. However, because her bad matchup are what you encounter at tournaments.
Still not on a proper keyboard, so this is the short version, but, yeah, Rosa is top 5. She can't be top 3 when other characters have better spreads vs the meta.
Now onto her advantageous matchups. She beats Mario, Diddy Kong, Fox, Sonic, hard counters Ness, and potentially beats Ryu. No other top tier can claim this many advantageous matchups against the rest of top tier (they generally have more even matchups, whereas I feel Rosa either loses or wins).
I haven't mentioned MK yet, because I'm not sure how relevant to the meta MK is right now. This is likely to be Rosa's second worst matchup though, so Rosa could potentially go down as more players pick up MK. Hasn't happened yet though, and the best US MKs don't seem as optimized as Leo or Abadango. Rosa also has some even or slightly disadvantaged matchups amongst B tier (ie. Yoshi, Ike, Peach, Olimar), but these characters are also less relevant to the meta. Considering Rosa is still getting great results just about everywhere, it suggests that her problem matchups don't impede her viability as a solo main. If people are using primarily top tier matchups and results to determine viability, I could see Rosa as third.