What is Three-key exposition?

In music, the three-key exposition is a particular kind of exposition used in sonata form.

Normally, a sonata form exposition has two main key areas. The first asserts the primary key of the piece, that is, the tonic. The second section moves to a different key, establishes that key firmly, arriving ultimately at a cadence in that key. For the second key, composers normally chose the dominant for major-key sonatas, and the relative major for minor-key sonatas. The three-key exposition moves not directly to the dominant or relative major, but indirectly via a third key; hence the name.