The Manx Flag

The Manx flag bears the famous Trie Cassyn (three legs) of Man. They are generally believed to have been introduced to the Island about 1266 by Alexander III of Scotland, who expelled the Norse Vikings from the Island.
One theory suggests that Alexander replaced the Norse ship emblem with the three legs of Sicily, with whose royalty he had close family connections. The symbol of the conjoined three legs had been used in Sicily from ancient times, but the armour and spurs may have been added by Alexander.
Others believe the three legs derive from a similar device popular amongst the Celts and Norsemen of north-west Europe.
Some Manx folk tales connect Mannanan, the Celtic Sea God who lived on the island, with the three legs or a fiery wheel, but these stories may have been invented after the adoption of the Island's three legs emblem.
The correct heraldic description is:
"Gules, three armed legs proper, conjoined in fess, at the upper part of the thigh, flexed in triangle, garnished and spurred topaz, with the motto 'Quoconque jeceris stabit', surrounding it in a garter."
The Latin motto, first recorded in 1688, translates as, "Whichever way you throw me I shall stand."

Sources:
Guide to the Isle of Man, 9th edition, published 1950 by Ward Lock & Co., Ltd., London and Melbourne.
1987/88 Directory of the Isle of Man, published by Manx Life Ltd., Douglas IoM.