TL;DR: Three Houses set the table.  Three Hopes brought the dessert.  Well worth it for either type of fan.

Three Hopes is basically a "what if" story.  In Three Houses, the three future leaders encounter Kostya, who has been assigned to take them out.  But what if that never happened?  How would their stories change?

That's what Three Hopes is doing.  But it's much more than that.  Characters and segments are fleshed out far further than from Three Houses' point of view, and it is true that there's a bit of retcon going on.

Don't expect strategy, at least not the 'normal' kind.  It's a Musou game in its finest form.  (Can you believe, there are people out there that don't know what "Musou" means?)

What I Liked

✅ As a Musou, the gameplay is smooth and the stages don't overstay their welcome.
✅ As a Fire Emblem(ish) game, the weapon strength/weakness mechanic is used decently well, and adds a bit of variety to the gameplay.
✅ Fully voiced cutscenes with full English dub all around - and a LOT of it.
✅ Characters that weren't fully fleshed out in Three Houses get more backstory here.

What I Didn't Like

👎 Rhea wasn't playable.
👎 Many of the rewards quests are just tedious busywork.
👎 Required stages with silly S-rank requirements.
👎 Stages where you have to stop escape yet are positioned on the other side of the map.
👎 No Hegemon Husk battle.
👎 Edelgard's route (Scarlet Blaze) is more boring than it should be.

If you haven't played Three Houses, much of the game plot and story won't make any half sense, but if you're a Musou fan, that's usually not a hard requirement.  If you did play Three Houses and enjoyed it for what it was, this is as close to a sequel as you're going to get.  Just don't expect the same level of support interactions.