To say that Fire Emblem Three Houses is a disappointment is the understatement of a century. There is a solid foundation of a setting and potential ambitious narrative pieces to be told here but almost everything surrounding it's execution is beyond dreadful. First things first, it does not understand how to handle it's own story structure. There are 4 routes, 5 if you play the DLC, but two of those routes are utterly worthless and retread the exact same plot threads, events and themes with nothing but slightly different endings which even then just comes down to one of them having a cutscene chopped in half. Silver Snow and Verdant Wind are the exact same story with a different lord that does the exact same thing anyway. On top of this, there's a ton of artificial padding from them not being able to balance the pacing of the gameplay and the story. The Monastery seems cool at first but when you realize that there's nothing to do across the 200+ hours this game will take you, you get so burned out of the social aspect of this game and how worthless most of it is. Especially when almost everything in the Monastery, besides recruiting which only matters for pre-skip anyway, is accessible in menus outside of it and other things like character dialogue or cooking can only be done once every 4 times you visit. And although this is a Switch game that I don't expect The Witcher or Elden Ring level visuals, this game is extremely ugly to look at. Half of the character designs are poor, the art style is poor, there is a HILARIOUS lack of portraits that detract from the story, the graphics look horrible, the colours are bad and the art direction/architecture is very uninspired.
Gameplay, for a strategy RPG, is genuinely pathetic. I have played visual novels with better gameplay balancing and design than this. Maps in this game are utterly worthless. There's about 10-15 at a stretch that are already just reskins with barely anything unique about them scenario wise or mechanically then to get rehashed to oblivion being used in quite literally every kind of battle to the point that you get sick of all of them by the time you're done with your first route. That is genuinely pathetic.
I also think that this is the first game I've played where permadeath genuinely does not matter. The units, both allied and enemy, are so poorly balanced on any difficulty that they game just becomes "send Byleth and your Lord to solo it" after you reach about level 10. Every enemy is a straight up joke due to the freedom you get.

Narrative wise, Three Houses flops between barely passable with some good characters and moments scattered between to dreadfully pathetic. The world building is poor. Things integral to a lot of the themes they attempt to showcase aren't fleshed out at all like the class system, social systems and the general status of each nation's commonfolk as well as how they're affected by the events.
Verdant Wind is supposed to be focused on geopolitics in the midst of a civil war as well as overcoming xenophobia except other nations exist in name only. They have absolutely zero impact on the plot of the route whatsoever and only ever get brought up in support conversations and a single Paralogue. On top of that, the actual main theme and goal is never actually achieved in the story making the whole thing incomplete. They never actually get to a point to solve or even confront the issue of geopolitics and xenophobia. They just throw it in a single sentence summary before the credits.
Azure Moon is a step up but suffers in it's own way. It's repetitive, poorly paced and is full of plot conveniences that serve to defecate on the themes and subject matter present in this route along with horrendous gameplay/narrative cohesion that they abide by or ignore at their own convenience really making integral turning points of the story not hit at all.
Silver Snow I can't even justify as it being a separate route. It is quite literally Verdant Wind with a few extra cutscenes tacked onto it. There is no reason why the two had to be separate routes besides just being a lazy way to pad out the game to make it seem even bigger and more ambitious.
Crimson Flower is the route with the most drastic change but is also the one impacted the worst by the lazy writing. It's the shortest, the one with the most conveniences and probably the laziest one in terms of writing. The entire concept that this route stands on is misrepresenting the others to try and parade itself as a "true" or "correct" route while also doing everything it can to avoid confronting it's subject matter head on in a thought provoking way. It honestly feels like the Lord's romanticized echo chamber more than anything else. On top of this, there are numerous plot holes created by the ending either not explaining certain plot elements or just outright contradicting itself.

Characters are a mixed bag which is unsurprising coming from a cast as big as this one and a game with writing as clumsy. The good characters are very good with great interactions and supports with good thematic exploration but generally have no place in the story because they're "expendable;" made to be removed with as little impact as possible due to permadeath. The great ones that I personally enjoyed were Linhardt, Lysithea, Seteth, Flayn, Sylvain, Felix, Cyril and Hilda. Some were pretty middling despite great setup like Ingrid who dwells on the waifubait aspect of her character far too much in her supports and Paralogue. The bad ones are horrible. Marianne and Bernadetta are genuinely insulting. Bernadetta makes a complete joke out of her subject matter while Marianne completely trivializes it in the most insulting way possible.
The Lords aren't really mixed though as I didn't like any of their characters. They all had potential but were extremely underexplored. Dimitri's character is lazy and the depiction of his mental health issues are frankly terrible from both a narrative and dialogue standpoint. The potential dynamic with Edelgard is pretty solid but Byleth, and ironically, Edelgard get in the way of it. All of his other dynamics and supports besides Dedue are very incomplete.
Claude is probably the Lord with the most potential but also the least explored. He's characterized as charismatic and shrewd with a very particular social detachment as the game's token master tactician but none of this is actually elaborated on or confronted. He does a few morally detached things in search for his goals but we never actually see his goals to fruition and they're never actually made a talking point.
Edelgard is...a thing. And not a good one. The main story bends over backwards to remove her agency and accountability to anything she does making her seem like a bumbling idiot but also glorifying her and, to put it crudely, sucking her off at any opportunity. Never letting any of her morally reprehensible actions actually be morally reprehensible because there must always be someone else to blame whether it's the victims or the people she's fighting against. Her trauma isn't explored they just make a point that she HAS trauma that drives her with very little actual elaboration, exploration or anything to help her progress and overcome her trauma.
Rhea is honestly a solid character in anything but Crimson Flower which very much mischaracterizes her in an attempt to drive Edelgard's point across. She's morally reprehensible but they do actually confront that and make it a talking point in routes she gets a direct focus while giving her a bit of nuance.

All in all, Fire Emblem Three Houses is a mess of a a game that has potential but suffers from very lazy/sloppy writing and extremely poor game design that has very little effort put into it. I went into this hoping for a great time but came out of it with very little to like. Do yourself a favor and play a good SRPG like Valkyria Chronicles or even Utawarerumono. If you enjoy lazy world building, mostly half assed character writing/drama, terrible game design and the most underexplored themes I've experienced in a singleplayer game, you'll like Fire Emblem Three Houses.