From Where is Your God by Michael Paul Gallagher, SJ.
My favorite example is in the stonework of Autun Cathedral where the three are in the same bed under a large blanket, and all are wearing their crowns! An angel is waking them to point at the star. One of them is shown with eyes wide open in wonder, another half-awake, but the third remains sound asleep - as if to represent the three stages of spiritual alertness in the medieval tradition.
In all the early tradition they were protrayed as identical figures. It is only from the twelft-century onwards that the magi-kings assume individual characteristics, being depicted as the three ages of life, or as representing different races and continents of the world. from this epoch comes the delighful legend that the three met for Christmas Mass in Armenia in AD 54 and that they died happily within a few days, all being well over a hundred years old. At some point their supposed bodies showed up in Milan, but after the sack of that city in 1164, Cologne Cathedral managed to acquire the relics and they remain there still in a magnificent enamelled shrine. In the late Middle Ages a Cologne breviary announced that the kings had in fact been consecrated bishops by St Thomas in India, thereby creating another problem for artists concerning what headgear to give them.
Apart from the legends and the art, spiritual writers reflected in the Magi in many ways. The Venerable Bede was one of those who suggested meanings for the three gifts: gold signified kingship, incense divinity, and the myrrh was a prophesy of the Passion. A few centuries later St Bernard's interpretation was more down to earth: money for the poor family; incense to disinfect the stable; and myrrh as a herbal remedy against worms in children.