Sounds like [*The Innkeeper's Song*][1] (1993) by Peter S. Beagle, mainly by dint of one of the characters being magically disguised as a woman. There is also a stableboy in the story.

Here is the logline: "Three powerful women (each with her own secret past), a stable boy, a weaver's son, and an innkeeper set in motion a series of events that brings each of them face to face with the forces of magic and the workings of fate."

[![Three women, one black, one white with black hair dressed in white, and one hooded. Cover art by Palencar.][2]][2]

  [1]: https://www.goodreads.com/book/show/11938.The_Innkeeper_s_Song?from_search=true
  [2]: https://i.sstatic.net/2jUAF.jpg