Three is an auspicious number. Things grouped in threes are believed to acquire prestige by virtue of the number's magic. Likewise, a ritual action repeated three times is considered to bring good luck.
For example, in a village festival, the priest leads worshippers three times round the shrine precinct while praying for the fulfillment of their wishes. The bride and bridegroom in a Shinto wedding drink sake from a set of three cups, sipping from each one three times.
From ancient times, there has also been the custom of celebrating in threes precious or rare things, people and places. For example, there are three calligraphers designated as the nation's best -- Emperor Saga, Tachibana-no Hayanari and Kobo Daishi, all of the Heian Period -- as well as three scenic spots -- Ama-no Hashidate (a bay on the coast of the Sea of Japan), Itsukushima (today known as Miyajima, in Hiroshima Bay) and Matsushima (a bay northeast of Sendai on the Pacific Coast). Of course, opinions sometimes differ as to which three should be designated as best.