IIRC, the UA Cavalier was largely focused on riding but the Knight & Samurai were "noble educated warriors".
I don't know what were the feedback results, but personally I thought the Cavalier was appalling for two reasons:
- riding abilities are always problematic in D&D because of many adventures being in dungeons (or anyway in places where a horse is impractical), leading to player's frustration for having abilities she cannot use, and DM's pressure to adapt adventures to compensate
- the UA version re-used the Superiority Dice mechanics but basically just offered a small subset of maneuvers from the Battlemaster, plus maybe 1-2 new out-of-combat unique maneuvers
In my opinion Knight & Samurai were a more solid concept, although the mechanics weren't particularly interesting, and probably it would make a lot of sense to merge these into a single subclass, and just let flavor distinguish between the European and Asian versions. However, I guess a lot of people found both of them weak concepts given the fact that any Fighter with the Noble background can adequately cover them.
So while my preference would be to merge all the three of them together, it's really possible that the Cavalier is just the horse-riding focused subclass, but hopefully with better mechanics.