That type of object is known as a tryptich, from the Greek to mean three-fold. They are an incredibly common phenomenon throughout the medieval period across Europe, particularly in works of devotional art, and could range in size from giant cathedral altarpieces to something the size of a matchbox that could be carried in your pocket.

Apart from the obvious Christian symbolism of being three things in one, tryptichs were popular because their format allows them to be easily packed away for safety, security or to make them easily portable. In cases of mass popular worship, this functionality may have served to add a further element of spectacle to religious ceremonies heavily based around visual pomp. In cases of private worship, a tryptich could be easily packed away after a ceremony.