Triptych
A work of art that is separated into three panels. The middle panel is the largest, with two small panels on other side. The side panels might fold in.
Why I Looked It Up
I was reading a book about contemporary art called The $12 Million Stuffed Shark, and it used the term to describe the titular artwork (pictured.)
Postscript
Added on
From What's Bred in the Bone:
The picture was a triptych, of which the central panel was five feet square, and the two flanking panels were of the same height, but only three feet wide.