quocunque jeceris stabit - Latin generally translated as whichever way I am thrown I will stand - see http://www.isle-of-man.com/manxnotebook/history/legs.htm for some history of its first appearance.
However the verb is I think 'iaceo' to lie (my latin stopped 45 years ago and I only have the shorter dictionary to hand - normally this very means to lie flat but also has usage to be refuted which I think leads some weight to my theory that it actually means however the coin is tested it will pass (firm, stable etc) - because the 3 legs were a much older device the tag being on all coins along with the legs from 1688 began to be associated with legs and not the coin.

The fairy bridge is as most know a late Victorian invention specially for tourists - that the manx had a belief in 'little people' is well attested but I suggest not quite in same prityfied way as generally portraied today.