From William Studwell's The Christmas Carol Reader:
It was all the fault of the gold, frankincense, and myrrh. From that brief mention of the gifts presented by the Wise Men to the child Jesus, as related in Matthew 2:11, evolved a very persistent and pervasive bit of religious folklore. The fine and expensive gifts, in the logic of the myth, meant that the visitors had to be of very high station, such as kings. The same logic pattern determined that the three types of offering had to correspond with the number of persons involved. Accordingly the erroneous but basically harmless illusion of "the Three Kings" has become a deeply embedded imperfection of our annual observance of Christmas.
The strong perpetuation of the myth by "We Three Kings of Orient Are," the best-known carol on the theme of the Wise Men, has done absolutely nothing to help the situation. But Pittsburgh-born John Henry Hopkins, Jr., (1820-1891) probably did not at all reflect on the theological implications of his little song when he wrote both words and music in 1857. At the time the versatile clergyman, author, journalist, book illustrator, and designer of stained glass windows and other ecclesiastical objects, and was working as editor of the Church Journal in New York City. His only apparent purpose was to devise a special Christmas present for his beloved nephews and nieces. Annually, Hopkins made a holiday trip to the Burlington area of Vermont, which was the home of his father John Henry Hopkins, Sr., (1792-1868), the long-time Episcopal bishop of that state. As usual, bachelor Uncle Henry did not disappoint the children, for the dramatization of the story from Matthew was reportedly a big hit in the Hopkins household.
The success of "We Three Kings" within the author's family circle was soon replicated in the outside world. As early as 1859, the song may have been put into print. Although the 1859 date is uncertain, by 1865 it had definitely appeared in the literature two times, first in Hopkins' 1863 collection, Carols, Hymns, and Songs, and subsequently in a separately published, specially illustrated 1865 version. Both the 1863 and 1865 publications carried the variant title "Three Kings of Orient." This rapid sequence of publication no doubt reflected the quickly spreading fame of Hopkins' carol, which ultimately became one of the most famous of all Christmas pieces. Because of this enduring musical amusement, and to a lesser extent another notable carol, "Gather Around the Christmas Tree," Hopkins has received a microniche in history and the appellation "Vermont's Father Christmas." Such a dubbing is geographically inappropriate, for Hopkins neither was born in that state nor did he create his carol there.