The triple spiral, or triskelion, is typically considered a Celtic design, but also has been found in some Buddhist writings. It appears in a variety of places as a three-faceted spiral, three interlocking spirals, or other variations of one shape repeated three times. One version is known as the Three Hares triskelion, and features three rabbits interlocked at the ears.
This symbol appears in many different cultures, and has been discovered as far back as on Lycaean coins and pottery from Mycaenae. It is also used as the emblem of the Isle of Man, and appears on regional banknotes. The use of the triskele as a symbol of a country is nothing new, though -- it has long been known as the symbol of the island of Sicily in Italy. Pliny the Elder connected the use as Sicily's emblem to the shape of the island itself.
In the Celtic world, the triskele is found carved in Neolithic stones all over Ireland and western Europe. For modern Pagans and Wiccans, it is sometimes adopted to represent the three Celtic realms of earth, sea and sky.
If you're interested in following a Celtic Pagan path, there are a number of books that are useful for your reading list. Although there are no written records of the ancient Celtic people, there are a number of reliable books by scholars that are worth reading: Celtic Reading List.
In addition to the complex Celtic knotwork often seen, Ogham symbols are found and used in a number of Celtic Pagan paths. Although there are no records of how Ogham symbols might have been used in divination in ancient times, there are a number of ways that they can be interpreted: Make a Set of Ogham Staves.