Part of it is that Three houses's cast is actually filled with characters with their own hopes and dreams and personalities.
The other FE casts just kinda exist to be cannon fodder for the main character.
That is just untrue.
While they have different degrees of success at, every Fire Emblem game tries to make its characters interesting enough for permadeath to actually mean something beyond losing a unit. Awakening and Fates in particular have Paralogues which take a deeper dive into some characters and the children mechanic, love it or hate it, helped personalize your experience by letting you play matchmaker with whoever you wanted.
Three Houses and Three Hopes are without a doubt the best cast in the series but that's because getting to know more about said cast has never been as easily accessible in a Fire Emblem game before or even
since these two games came out. You don't need high support ranks, you don't need to make them make babies with one another, they just need to be recruited in your party and nothing else (though recruiting some people can be a tall order in itself).
And on top of that,
every playable character gets to be part of at least one, including the DLC. That's something Awakening and Fates actually didn't manage to do and something Engage didn't even
try.
I don't know what version of the game you were playing but three houses had loads of lovable characters. and a few hateable
I would say there are a few, particularly the house leaders, who can only be enjoyed if you played their route. If all you've ever played is Azure Moon, you have no reason to find Edelgard likeable because you never saw a single one, for example.
Well, there's the childhood flashbacks but given how she acts in the present, they don't bring much for you to like the older Edie.
Some of the more pivotal characters are in a surprisingly gray area where they're written without the explicit intend to be likeable unless you specifically play the route where they have the biggest impact. Things aren't always black and white in Fodlan; people can have valid reasons to dislike someone and maybe they don't have the free time to see things your way by getting through the entire game another time just to see the good parts of someone.
And that's fine. I know I'd personally never force someone to play an entire whole ass game just because I think Dedue is neat because I sure as hell would be a hypocrite since I have a good enough amount of games in my backlog that I don't wanna commit to playing every route on a 5 year old game.
Three Hopes handling of its mysteries is one of the most baffing parts of its storytelling. You give us a very likeable protagonist with a past with a lot of interesting questions you could answer, and we got none of them answered, at least not in a satisfactory way. What makes it worse is they're tied to another character with their own set of questions that are also tied to a group in dire need of expansion, and they also get little explained. It felt like being mysterious for the sake of being mysterious, and as a result pointlessly held back their characters.
So ****ing true. I still huff the copium of a potential DLC but at this point I'm running out of copium