It would be ridiculous for those three to be the only schools of magic in the world, so I'm assuming that there are other schools of magic, but they're just not mentioned.

Harajuku magic school FTW!

... What?

The only Japanese Magic School I'm even aware of is the one from Negima. lol

There are more wizarding schools in the world, though Ms. Rowling does not mention very many of them. In The Goblet of Fire, Ron talks about how his older brother Bill had a pen-pal in Brazil who went to a wizarding school there that wanted him to do an exchange program with him (though the Weasley family could not afford it, so the pen-pal got offended and send Bill a cursed hat that made his ears shrivel up).

The fourth book also mentions Harry, Ron and Hermione seeing a tent at the Quidditch World Cup as they walk to get water with a banner stating "Salem Witches' Institute", which is a wizarding school in America.

Also, in The Tales of Beetle the Bard, there is mention of a school called "Wizarding Academy of Dramatic Arts" that is located in Britain, a school where witches and wizards can go if they seek a theatrical career.

So, there you have it!

-Nukaya

That satisfies my curiosity. Thank you.