This is a tricky one. The three are definitely different, yet sometimes can be considered the same. St. Nicholas was a fourth-century Turkish bishop who spent his life giving money to the poor, and it's said one of his favored methods was secretly leaving money in people's stockings overnight. Nicholas died on Dec. 6, and was eventually proclaimed a saint. Thus, Dec. 6 became known as St. Nicholas Day. Various cultures celebrated by instructing their kids to leave out stockings or shoes the night before so "St. Nick" could fill them with gifts like fruit, nuts and candy. [source: Why Christmas].
By the 16th century, Europeans were turning away from the idea of St. Nicholas, yet they loved the gifting tradition. So St. Nick morphed into a guy named "Father Christmas." First mentioned in 15th-century writings, he was a partying dude associated with drunkenness and holiday merrymaking. In the U.S., St. Nick became Kris Kringle. Father Christmas and Kris Kringle generally brought gifts on Christmas, not Dec. 6. When Dutch settlers began emigrating to the U.S., they brought with them stories of St. Nicholas, whom they called Sinterklaas. Soon Sinterklaas became Americanized as Santa Claus [sources: Religion Facts, Why Christmas].
By the 20th century or so, all of the Father Christmases, Kris Kringles, etc. became "Santa Claus," uniformly depicted as a round-bellied, white-bearded old guy who brings gifts on Christmas Eve or Christmas Day. Yet some people around the world, namely Christians from European countries where St. Nick was a beloved hero, still celebrate St. Nicholas Day on Dec. 6 by setting out shoes or hanging stockings the night before. So while Father Christmas and Santa Claus are definitely now one and the same, St. Nicholas is still a toss-up, with some people recognizing him as a distinct individual and others lumping him in with the other gift-bearing men [sources: Religion Facts, Why Christmas].