I actually ended up replaying all 3 very recently.
Birthright is pretty mediocre. It's a slow enemy-phase game with most maps being rout, and higher difficulties just sending waves of low quality enemies at you. The story is very bare basic, but the least offensive of the trilogy. It's also very diluted, intentionally introducing plot points that it ignores so that it can be covered in another game. From a purely gameplay perspective, I think it's better than Awakening at least.
Conquest is pretty good. Varied map design, rewards player-phase aggression, and difficulty comes in the form of giving enemies some threatening skills, which is the most interesting way to handle the difficulty compared to the other two. I think conquest strikes a great balance between being somewhat limited like old Fire Emblem games, while still having a lot of flexibility with it's class system. Story is very stupid with the plot constantly bending over backwards to make Corrin not the bad guy. Overall the best game of the bunch.
Revelation is rough. Maps are extremely gimmicky (some hits, some miss), unit balance is absolutely awful, and it provides a very unsatisfying conclusion to the joke of a story. Difficulty design is stat based, so rather than fight a bunch of low quality enemies like Birthright, you instead fight a few very strong enemies (which are about on par with your own units). I think this is a little more interesting than Birthright, but it ultimately encourages just funneling all your kills and resources into a select few units (primarily Corrin) and low manning the game.
Mechanically the games are very well designed, especially compared to the dogshit pair-up mechanics in Awakening. The problem is it lacks charm. Awakening wishes it had the map and mechanical design of Fates, and Fates wishes it had the story, world, and characters to lure you in like Awakening.
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