The triskele or triskelion symbol, which resembles three commas or teardrop shaped beads chasing one another round a circle, is a Buddhist meditational symbol that represents the three aspects of Dependent Relationship which give existence to all functioning things.
The Buddhist teaching on Dependent Relationship states that phenomena exist in three fundamental ways. Firstly, phenomena exist by dependence upon causes and conditions. Secondly, phenomena depend upon the relationship of the whole to its parts and attributes. Thirdly, and most profoundly, phenomena depend upon designation by the mind . The mind is a non-physical aspect of the universe that is not reducible to matter.
The appearance of motion of the three swirling teardrops symbolises that the impermanence of all compound phenomena arises from these three ever-changing relationships.
In traditional Buddhist art, triskeles are frequently seen in the centre of Dharma wheels, four-pronged vajras and auspicious symbol mandalas.
The triskelion magatama symbol is also a common feature of Celtic sacred art such as stone carvings, enamel ornaments, book illumination and knotwork.
This metaphysical symbol is probably the 'Caer Sidin' which was the object of meditation by the Druidic bard Taliesin, as he explains in this verse:
Mi a fum ynghadair flin
Uwch Caer Sidin
A honno ya troi fydd
Rhwng tri elfydd
Pand rhyfedd ir byd
Which is translated as:
I have presided in a toilsome chair
Over the circle of Sidin
Whilst that is continuously revolving between three elements;
Is it not a wonder to the world,
That men are not enlightened? 
The untranslated term 'Sidin' is not explained. Could it be related to the Sanskrit Siddhi ?
According to the article by John Michael Greer, the 'three elements' of Druid metaphysics to which Taliesin refers are known in old Welsh as Gwyar (change, causality), Calas (structure) and Nwyfre (consciousness).
An Indian Dharma wheel with a triskele at the hub
Japanese Buddhist Dharma wheel
Dharma wheel - Manjushri Center, Cumbria
Four pronged vajra with central triskelion (click to enlarge)
Manjushri Center, Cumbria
Manjushri Center, Cumbria
Celtic Triskele Symbols
Scottish Carved Stone 1
Scottish Celtic sculptured stone - inner and outer triskelions
Silver object (function unknown) from Saint Ninian's Isle Treasure, Shetland
Irish metalwork showing interleaved triskelions, second century A.D.
The Laxey Wheel Triskelion Isle of Man
In the three hares/rabbits triskelion, the aspect of motion is especially apparent, emphasising that all phenomena arise from the three dependencies and are thus inevitably impermanent and devoid of any essence.
This symbol was originally Buddhist, but travelled westwards along the silk routes and can be found in European medieval church ornamentation.
In Buddhism, as well as Druidism, mind is a primal aspect of reality that is not reducible to matter. For a discussion of why the mind is a non-physical, fundamental aspect of the universe which is not derived from anything else, see Confronting Materialism and the Delusion of the Mechanical Mind.
- Sean Robsville
Read more at Buddhist Philosophy
Mysterians, Mysterianism and the Mystery of the Mind
Cauldron, Chalice and Grail Symbolism in Buddhism and Celtic Wicca
Numinous Symbolism - Pagan, Buddhist and Christian
Celtic Buddhism - Buddhism in pre-Christian Britain
C J Jung, Buddhism, Tantra and Alchemy
RESOURCES FOR SYMBOLISM
 Victor Mansfield 'Time and Impermanence in Middle Way Buddhism and Modern Physics'. Talk at the Physics and Tibetan Buddhism Conference, University of California, Santa Barbara January 30-31, 1998
 Stewart, R. J. and Williamson, R. (1996) Celtic Bards, Celtic Druids , ISBN 0-7137-2563-X, publ Blandford, London, p 146.