Stone balls in the cemetery are almost always purely decorative. Cannonballs often ring veterans' gravesites. But if they occur in a group of three they could be a reference to gifts or money, like the three balls seen hovering over a pawnbroker's shop. The use of three balls to symbolize money comes from the legend of St. Nicholas of Myrna, who threw three sacks of money into the window of a poor man's house to allow him to provide dowries for his three daughters. St. Nicholas is also said to be a purveyor of secret gifts for children. In this context he is better known as Santa Claus.
Taken from Stories in Stone: A Field Guide to Cemetery Symbolism and Iconography written and photographed by Douglas Keister, published by Gibbs Smith, Salt Lake City, 2004.