About the 3 horse achievements.

[lid]

If I already have 4 mounts, which is too many, will I not be able to get the Moon Child, Matsukaze, and Persian Mare achievements? Or does that even matter?

[Tai_MT]

You must have less than three horses to get them.

That's what I read.

And that's how I got them.

[Zef]

If you take over a fief with the shop in you can sell mounts, so if you find that you have four just go there are sell however many you want. It takes up one order though, so you will need to choose wisely whether or not to go there.